Electrical field stimulation of eukaryotic cells

ABSTRACT

Methods of identifying activators and inhibitors of voltage-gated ion channels are provided in which the methods employ electrical field stimulation of the cells in order to manipulate the open/close state transition of the voltage-gated ion channels. This allows for more convenient, more precise experimental manipulation of these transitions, and, coupled with efficient methods of detecting the result of ion flux through the channels, provides methods that are especially suitable for high throughput screening.

CROSS-REFERENCE TO RELATED APPLICATIONS

The subject application is related to co-pending provisional application No. 60/304,955, filed Jul. 12, 2001, to which priority is claimed under 35 USC § 119(e).

STATEMENT REGARDING FEDERALLY-SPONSORED R&D

Not applicable.

REFERENCE TO MICROFICHE APPENDIX

Not applicable.

1. Field of the Invention

The present invention is directed to methods and associated apparatuses for stimulating eukaryotic cells by the application of electric fields. The electric fields are produced by certain arrangements of electrodes that create an electric potential difference in the environment of the cells, resulting in a change in membrane potential of the cells. The change in membrane potential affects various physiological processes within the cells, including the opening and closing of voltage- gated ion channels. The ability to alter the open/close transitions of voltage-gated ion channels by the application of electric fields as described herein provides for novel methods of screening compounds for the ability to modulate the activity of voltage- gated ion channels.

2. BACKGROUND OF THE INVENTION

Certain molecular events in eukaryotic cells depend on the existence or magnitude of an electric potential gradient across the plasma (i.e., outer) membrane of the cells. Among the more important of such events is the movement of ions across the plasma membrane through voltage-gated ion channels. Voltage-gated ion channels form transmembrane pores that open in response to changes in cell membrane potential and allow ions to pass through the membrane. Voltage-gated ion channels have many physiological roles. They have been shown to be involved in maintaining cell membrane potentials and controlling the repolarization of action potentials in many types of cells (Bennett et al., 1993, Cardiovascular Drugs & Therapy 7: 195-202; Johnson et al., 1999, J. Gen. Physiol. 113: 565-580; Bennett & Shin, “Biophysics of voltage-gated sodium channels,” in Cardiac Electrophysiology: From Cell to Bedside, 3^(rd) edition, D. Zipes & J. Jalife, eds., 2000, W.B. Saunders Co., pp. 67-86; Bennett & Johnson, “Molecular physiology of cardiac ion channels,” Chapter 2 in Basic Cardiac Electrophysiology and Pharmacology, 1^(st) edition, A. Zasa & M. Rosen, eds., 2000, Harwood Academic Press, pp. 29-57). Moreover, mutations in sodium, calcium, or potassium voltage-gated ion channel genes leading to defective channel proteins have been implicated in a variety of disorders including the congenital long QT syndromes, ataxia, migraine, muscle paralysis, deafness, seizures, and cardiac conduction diseases, to name a few (Bennett et al., 1995, Nature 376: 683-685; Roden et al., 1995, J. Cardiovasc. Electrophysiol. 6: 1023-1031; Kors et al., 1999, Curr. Opin. Neurol. 12: 249-254; Lehmann et al., 1999, Physiol. Rev. 79: 1317-1372; Holbauer & Heufelder, 1997, Eur. J. Endocrinol. 136: 588-589; Naccarelli & Antzelevitch, 2000, Am. J. Med. 110: 573-581).

Several types of voltage-gated ion channels exist. Voltage-gated potassium channels establish the resting membrane potential and modulate the frequency and duration of action potentials in neurons, muscle cells, and secretory cells. Following depolarization of the membrane potential, voltage-gated potassium channels open, allowing potassium efflux and thus membrane repolarization. This behavior has made voltage-gated potassium channels important targets for drug discovery in connection with a variety of diseases. Dysfunctional voltage-gated potassium channels have been implicated in a number of diseases and disorders. Wang et al., 1998, Science 282: 1890-1893 have shown that the voltage-gated potassium channels KCNQ2 and KCNQ3 form a heteromeric potassium ion channel known as the “M-channel.” Mutations in KCNQ2 and KCNQ3 in the M-channel are responsible for causing epilepsy (Biervert et al., 1998, Science 279:403406; Singh et al., 1998, Nature Genet. 18: 25-29; Schroeder et al., Nature 1998, 396: 687-690).

Voltage-gated sodium channels are transmembrane proteins that are essential for the generation of action potentials in excitable cells (Catterall, 1993, Trends Neurosci. 16: 500-506). In mammals, voltage-gated sodium channels consist of a macromolecular assembly of α and β subunits with the α subunit being the pore- forming component. α subunits are encoded by a large family of related genes, with some α subunits being present in the central nervous system (Noda et al., 1986, Nature 322:826-828; Auld et al., 1988, Neuron 1:449461; Kayano et al., 1988, FEBS Lett. 228:187-194) and others in muscle (Rogart et al., 1989, Proc. Natl. Acad. Sci. USA 86:8170-8174; Trimmer et al., 1989, Neuron 3:3349).

Voltage-gated calcium channels are transmembrane proteins that in the open configuration allow the passive flux of Ca²⁺ ions across the plasma membrane, down the electrochemical gradient. They mediate various cell functions, including excitation-contraction coupling, signal transduction, and neurotransmitter release.

Current methods of drug discovery often involve assessing the biological activity (i.e., screening) of tens or hundreds of thousands of compounds in order to identify a small number of those compounds having a desired activity. In many high throughput screening programs, it is desirable to test as many as 50,000 to 100,000 compounds per day. Unfortunately, current methods of assaying the activity of voltage-gated ion channels are ill suited to the needs of a high throughput screening program. Current methods often rely on electrophysiological techniques. Standard electrophysiological techniques involve “patching” or sealing against the cell membrane with a glass pipette followed by suction on the glass pipette, leading to rupture of the membrane patch (Hamill et al., 1981, Pflugers Arch. 391: 85-100). This has limitations and disadvantages. Accessing the cell interior may alter the cell's response properties. The high precision optical apparatuses necessary for micromanipulating the cells and the pipettes make simultaneous recording from more than a few cells at a time impossible. Given these difficulties, the throughput that can be achieved with electrophysiological techniques falls far short of that necessary for high throughput screening.

Various techniques have been developed as alternatives to standard methods of electrophysiology. For example, radioactive flux assays have been used in which cells are loaded with a radioactive tracer (e.g., ⁸⁶Rb⁺, ²²Na⁺, [¹⁴C]-guanidinium) and the efflux of the dye is monitored. Cells loaded with the tracer are exposed to compounds and those compounds that either enhance or diminish the efflux of the tracer are identified as possible activators or inhibitors of ion channels in the cells' membranes.

Assays that measure the change in a cell's membrane potential due to the change in activity of an ion channel have been developed. Such assays often employ voltage sensitive dyes that redistribute between the extracellular environment and the cell's interior based upon a change in membrane potential and that have a different fluorescence spectrum depending on whether they are inside or outside the cell. A related assay method uses a pair of fluorescent dyes capable of fluorescence resonance energy transfer to sense changes in membrane potential. For a description of this technique, see Gonzalez & Tsien, 1997, Chemistry & Biology 4:269-277. See also Gonzalez & Tsien, 1995, Biophys. J. 69:1272-1280 and U.S. Pat. No. 5,661,035. Other methods employ ion selective indicators such as calcium dependent fluorescent dyes to monitor changes in Ca²⁺ influx during opening and closing of calcium channels.

Ideally, methods of screening against voltage-gated ion channels require that the transmembrane potential of the cells being assayed be controlled and/or that the ion channels studied be cycled between open and closed states. This has been done in various ways. In standard electrophysiological techniques, the experimental set-up allows for direct manipulation of membrane potential by the voltage clamp method (Hodgkin & Huxley, 1952, J. Physiol. (Lond.) 153: 449-544), e.g., changing the applied voltage or injecting various ions into the cell. In other methods, changing the extracellular K⁺ concentration from a low value (e.g., 5 mM) to a higher value (e.g., 70-80 mM) results in a change in the electrochemical potential for K⁺ due to the change in the relative proportion of intracellular and extracellular potassium. This results in a change in the transmembrane electrical potential towards a more depolarized state. This depolarization can activate many voltage-gated ion channels, e.g., voltage-gated calcium, sodium, or potassium channels. Alternatively, Na⁺ channels can be induced into an open conformation by the use of toxins such as veratridine or scorpion venom (Strichartz et al., 1987, Ann. Rev. Neurosci. 10: 237-267; Narahashi & Harman, 1992, Meth. Enzymol. 207: 620-643). While sometimes effective, such experimental manipulations may alter the channel pharmacology, can be awkward to perform, and can lead to artifactual disturbances in the system being studied.

Electrical field stimulation of cells has been performed on a single cell by sealing a glass microelectrode to the cell membrane. Rupture of the sealed patch of cell membrane resulted in an electrical connection between the interior fluid in the glass microelectrode and the fluid within the cell that was used to stimulate the cell via an electronic pulse generator. The electrophysiological response of the cell was measured via a sensitive electronic amplifier. The disadvantage of this technique is that only one cell at a time was tested and it is a tedious and time consuming operation to seal the microelectrode to an individual cell.

HEK293 cells have been grown on a silicon chip made up of an array of field-effect transistors. Some of the cells were positioned over the gate region of the transistors, thus having portions of their plasma membranes overlying the source and the drain. When a patch pipette in such cells manipulated the intracellular voltage, Maxi-K potassium channels in the cells' plasma membranes were opened. This led to current flow in the region between the cells' membrane and the transistor. This current flow modulated the source-drain current, which could be detected by an appropriate device. The chip plus cells was said to have potential as a sensor and as a prototype for neuroprosthetic devices. See Straub et al., 2001, Nature Biotechnol. 19:121-124; Neher, 2001, Nature Biotechnol. 19:114.

SUMMARY OF THE INVENTION

The present invention is directed to methods of identifying activators and inhibitors of voltage-gated ion channels in which the methods employ electrical field stimulation of the cells via extracellular electrodes in order to manipulate the open/close state transitions of the voltage-gated ion channels. This allows for more convenient, more precise manipulation of these transitions, and, coupled with efficient methods of detecting ion flux or membrane potential, results in methods that are especially suitable for high throughput screening in order to identify substances that are activators or inhibitors of voltage-gated ion channels.

The present invention also provides apparatuses for use in the above- described methods. In particular, modifications of standard multiwell tissue culture plates are provided where the modified multiwell tissue culture plates have electrodes that can alter the transmembrane electric potential of cells in the wells of the plates, thus altering the ratio of open/close states of voltage-gated ion channels in the cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a top view of one embodiment of the present invention. This embodiment comprises a glass slide 1 in which or upon which are a gold positive electrode 2 and a gold negative electrode 3 spaced such that a gap 4 of about 25 μm to 100 μm exists between the electrodes. The electrodes together with spacers 5 (here shown as plastic strips) arranged generally at right angles to the electrodes define a series of wells 6 about 100 μm deep into which cells can be placed and/or grown. FIG. 1B shows a cross-sectional side view of the embodiment of FIG. 1A. In this embodiment, the identities of the positive and negative electrodes can be interchanged, if desired. The electrodes need not be made from gold; other conductive materials may be used. Also, the spacers need not be plastic; other non- conductive materials may be used.

FIG. 2A shows a top view of an embodiment of the present invention in which a typical 96 well plate contains electrodes within each well. FIG. 2B shows a cross-sectional side view of one of the wells in FIG. 2A. The well has a first electrode 1 (here shown as a positive electrode) on the side 2 of the well, a second electrode 3 (here shown as a negative electrode) on the bottom 4 of the well, a strip of an optional insulating material 5 on the bottom of the well, and a cell 6 at the bottom of the well. A single cell is shown merely for convenience of illustration; in most cases a plurality of cells would be in the bottom of the well. The sides 2 of the well are made of a non-conducting material such as plastic and the bottom of the well is made from a conducting material such as indium tin oxide (ITO). The well is shown with a fluid level 7 sufficient to completely cover the cell 6 and the second electrode 3 at the bottom 4 of the well and to reach the first electrode 1 on the side 2 of the well. The well is not drawn to scale with respect to FIG. 2A. FIG. 2C shows an alternative arrangement of electrodes in a well. In this embodiment, both the positive electrode 1 and the negative electrode 2 are in the bottom 3 of the well. In this embodiment, the sides 4 and bottom 3 of the well are made of non-conducting material such as plastic. The fluid level 5 is such as to cover the cells 6 as well as the positive 1 and negative 2 electrodes.

FIG. 3 shows a single well 1 from an embodiment of the invention where first 2 and second 3 electrodes are interdigitating and have been chemically etched on a layer of conductive material on the surface of a glass substrate 4. The well is generally circular with a 3 mm diameter. The electrodes are 10 μm wide and have a spacing of 160 μm. Either the first 2 or the second 3 electrodes may function as the positive electrode. The width of the electrodes and the spacing between the electrodes can be varied. The width is preferably between 1 and 10 μm; the spacing between the electrodes is preferably 5 μm to 160 μm. In particularly preferred embodiments, the spacing between the electrodes is at least as great as a typical diameter of a eukaryotic cell (i.e., about 40 μm to 50 μm).

FIGS. 4A and 4B illustrates an embodiment in which wells are formed by attaching a well frame onto the substrate. FIG. 4A shows an exploded view of the embodiment containing a well frame 1 the openings 2 of which form the wells on the substrate 3 where the well frame 1 is attached to the substrate 3 (e.g., by gluing it in place), a contact guide plate 5 with a spring loaded contact 6, and a printed circuit board (PCB) 7. The substrate holder 4 is used to hold the assembled device in position on a measuring instrument such as a microscope or fluorescent plate reader (not shown). The PCB 7 contains connections through which the electrodes (not shown) can be linked to a pulse generator (not shown). FIG. 4B shows an assembled view.

FIG. 5 shows an arrangement of interdigitating electrodes formed upon a substrate that contains virtual wells. Virtual wells are described further herein.

FIG. 6 shows a single well from an embodiment of the invention where two substantially parallel plates 1 have their opposing surfaces coated with conductive layers 2 between which is sandwiched a droplet of fluid containing the cells to be tested 3. One conductive layer is a positive electrode (here the upper conductive layer 4) while the other conductive layer is a negative electrode (here the lower conductive layer 5). Of course, the identity of the electrodes could be reversed, with the upper conductive layer being the negative electrode and the lower conductive layer being the positive electrode). In particular versions of this embodiment, the plates are glass and the conductive layer is indium tin oxide (ITO). The conductive layer preferably has a thickness of about 200 Å to 2,000 Å, or 500 Å to 1,500 Å, or 800 Å to 1,200 Å.

FIG. 7 shows a single well 3 from an embodiment of the invention where one of the electrodes is a thin coating of conductive material 2 on the surface of a flat substrate 1 and forms the bottom 10 of the well. The other electrode 7 enters the well 3 from above and makes contact with the fluid 5 within the well 3. Electrode 7 is shown in cut-away view. Electrode 7 contains a central conductive material portion 8 that is surrounded by an insulator 6. For the sake of simplicity, a single cell 4 is shown in the well. Generally, at least 10⁵ cells would be present in the well. The conductive layer preferably has a thickness of about 200 Å to 2,000 Å, or 500 Å to 1,500 Å, or 800 Å to 1,200 Å.

FIG. 8 shows a single well 4 from an embodiment of the invention where the bottom of the well 4 is a filter membrane 12 upon which cells can be grown. For simplicity, a single cell 8 is shown on the filter membrane 12. The well 4 is located in a trough 2 having a glass bottom 1 and filled with a first fluid 3. One electrode 7 enters the well 4 from above and makes contact with a second fluid 5 within the well 4. Electrode 7 contains a central conductive material portion that is surrounded by an insulator 6 and is connected to a pulse generator (not shown) by a first lead 9. A second electrode 11 is positioned within the first fluid 3 and is connected to the pulse generator by a second lead 10. The second electrode 11 is shown in cut-away view. The second electrode 11 actually forms a circle in the bottom of the well 4. Either the first electrode 7 is the positive electrode while the second electrode 11 is the negative electrode or the first electrode 7 is the negative electrode while the second electrode 11 is the positive electrode.

FIG. 9A shows a single well 2 from an embodiment of the invention where both the positive 5 and negative 8 electrodes enter the well 2 from above. The well 2 contains fluid 3 in which a single cell 9 is shown, although generally a plurality of cells will be present in the well 2. The positive electrode 5 is connected to a pulse generator (not shown) by a positive lead 6. The negative electrode 8 is connected to the pulse generator by a negative lead 7. Both electrodes are embedded in an insulator 4. The positive 5 and negative 8 electrodes traverse the interior of the insulator 4 such that the positive 5 and negative 8 electrodes are generally perpendicular to a glass plate 1 that forms the bottom of the well 2. However, when the positive 5 and negative 8 electrodes exit the bottom 10 of the insulator 4, the positive 5 and negative 8 electrodes are each bent into a 90° angle so that they lie on and parallel to the bottom 10 of the insulator 4. FIG. 9B is a view looking up from the glass plate 1 that forms the bottom of the well 2 and shows the arrangement of the bent portion of the positive 5 and negative 8 electrodes lying on bottom of the insulator 4.

FIG. 10A shows an embodiment of the invention where both the positive 5 and negative 8 electrodes enter the well 2 from above and the positive 5 and negative 8 electrodes are arranged in a manner similar to that of a co-axial cable. The positive electrode 5 is embedded in an insulator 4 with the negative electrode 8 coating the outside of the insulator 4. The positive electrode 5 is connected to a pulse generator (not shown) by a positive lead 6. The negative electrode 8 is connected to the pulse generator by a negative lead 7. The well 2 contains fluid 3 in which a single cell 9 is shown, although generally a plurality of cells will be present in the well 2. A glass plate 1 forms the bottom of the well 2. FIG. 10B shows a view looking up from below the positive 5 and negative 8 electrodes.

FIG. 11 shows an embodiment of the invention similar to the embodiment shown in FIG. 8 except that in FIG. 11 the electrode 7 that enters the well from above is not surrounded by an insulator but instead is within a pipette tip 6 and makes contact with a first fluid 5 also within the pipette tip 6 that is co-extensive with the first fluid 5 in the well 4. This arrangement has the advantage of minimizing the formation of bubbles in the first fluid 5 in the area at the end of the electrode 7. The bottom of the well 4 is a filter membrane 12 upon which cells can be grown. For simplicity, a single cell 8 is shown on the filter membrane 12. The well 4 sits in a trough 2 having a glass bottom 1 and filled with a second fluid 3. Electrode 7 is connected to a pulse generator (not shown) by a first lead 9. A second electrode 11 is positioned within the second fluid 3 and is connected to the pulse generator by a second lead 10. The second electrode 11 is shown in cut-away view. The second electrode 11 actually forms a circle in the bottom of the well 4. Either electrode can be the positive or negative electrode.

FIG. 12A-B shows an embodiment that is similar to the embodiment of FIG. 7 in having one electrode enter from above while the other electrode forms the bottom of the wells. FIG. 12A is a side cross-sectional view that shows a substrate that is a 96-well microtiter plate in which one electrode 1 is a layer of a conductive material such as ITO that forms the bottom of the wells 2. The other electrode 3 enters the wells from above and makes contact with the fluid in the wells (fluid not shown). The electrodes are connected to an electrical pulse generator 4 by leads 5. Either electrode may be the positive or negative electrode. An alternative embodiment, similar to that shown, is to replace the bottom of standard 96, 384, 1536, or 3456 well plates with a conductive material such as ITO, which forms one electrode. The second electrode is lowered into each well from above. Contact to the ITO electrode can be made via electrically conducting silver epoxide or by placing a 3 M KCl (or similar salt solution) in alternate wells as the contact to the ITO bottoms from a platinum wire. FIG. 12B shows a top view of the substrate.

FIG. 13A-B shows an embodiment comprising two multiwell substrates containing virtual wells. FIG. 13A is a side cross-sectional view that shows the top substrate 1 approaching the bottom substrate 2. The top electrode 3 is made of a conducting material such as ITO and forms the bottom of the virtual wells 4 of the top substrate 1. Similarly, the bottom electrode 5 is made of a conducting material such as ITO and forms the bottom of the virtual wells 6 of the bottom substrate 2. A thin layer of TEFLON® or a similar hydrophobic material 11 covers the surfaces of the conducting material on the substrates. Circular areas of the surface of the substrate that lack TEFLON® are relatively hydrophilic and form the virtual wells. The TEFLON® layer is about 0.5 μm to 100 μm thick. The top 3 and bottom 5 electrodes are connected to an electrical pulse generator 6 by leads 7. The left most wells of the apparatus are shown containing fluid drops. The top drop 8 might contain a substance such as a drug or a compound to be tested while the bottom drop 9 might contain cells expressing a voltage-gated ion channel. FIG. 13B shows the apparatus after the top 1 and bottom 2 substrates have moved close enough together so that the top 8 and bottom 9 drops have mixed. 10 is a spacer (not shown in FIG. 13A) that helps to align the top 1 and bottom 2 substrates and keeps the substrates the proper distance apart for mixing of the drops.

FIG. 14 illustrates the principles of electrical field stimulation of cells.

FIG. 15 shows two wells from an embodiment where one electrode enters the wells from above 1 while the second electrode is formed from the transparent ITO-coated bottom 2 of the transparent substrate 3 that is in contact with a highly conductive metal grounding grid 4. The dashed lines with arrowheads illustrate how current flows from the electrodes that enter from above 1 through a buffered salt solution 5 and the cells 6 and through the ITO layer 2 and the metal grounding grid 4. Arrows 7 within the substrate 3 illustrate how light from a source used in the detection system (not shown) would pass in the upward direction through the transparent substrate 3 and the ITO layer 2 into the cells 6 and then be re-emitted by the cells 6 as fluorescence and pass downward to a detector (not shown). Optional adhesive seals 8 that can be used to attach the wells to the ITO-coated substrate 3 are shown. The thickness of the ITO layer is preferably about 200 Å to 2,000 Å, or 500 Å to 1,500 Å, or 800 Å to 1,200 Å.

FIG. 16A shows two wells of a multiwell embodiment having a conductive layer 1 such as ITO that forms the bottom of the wells. The positive electrode 2 enters the left well 3 from above while the negative electrode 4 enters the right well 5 from above. The transparent layer of a conductive material 1 such as ITO coats a transparent substrate 7 such as glass. The dotted line with an arrowhead shows the path of current flow. Of course, the identity of the positive and negative electrodes could be reversed. Cells 8 are shown in fluid 9 within the wells. Optional adhesive seals 10 that can be used to attach the wells to the ITO-coated substrate 7 are shown. Light path is indicated by arrows in the substrate. FIG. 16B shows a side cut-away view of this embodiment that illustrates how the positive 2 and negative 4 electrodes might be connected to a pulse generator 11. Also shown is the transparent conductive layer 6 coating the transparent substrate 7. FIG. 16C shows a top view of the embodiment that illustrates the alternating pattern of positive and negative electrodes. FIG. 16D is a photograph of this embodiment that has been partially disassembled. The wells are formed by a well frame 12 that is attached to the glass substrate 13 that is has been coated with ITO. During normal operation, the substrate will cover all the wells. For the purpose of illustration, this view shows only part of the substrate.

FIG. 17 shows a graphical representation of data obtained from an embodiment of the invention similar to that depicted in FIG. 16. The data represent Ca²⁺ influx into HEK293 cells that have been transfected to express the human α1H T-type voltage-gated calcium channel (GenBank accession no. AF073931). Ca²⁺ influx occurred when the T-type channels opened and was measured by detecting fluorescent emission at 520-560 nm of the calcium indicator dye Fluo4 that had been excited at 480 nm. At the time points indicated, a preselected voltage was applied through the electrodes. This resulted in the opening of a portion of the T-type channels, allowing Ca²⁺ influx. This caused a spike in the fluorescent emission at 520-560 nm by the calcium indicator dye Fluo4. The spice gradually decayed, as shown.

FIG. 18A-B shows a nucleotide sequence encoding the human PN3 sodium channel (SEQ.ID.NO.:1). FIG. 18C shows the corresponding amino acid sequence (SEQ.ID.NO.:2). From GenBank accession no. AF117907.

FIG. 19A-B shows a nucleotide sequence encoding the α1H subunit of the human T-type calcium channel (SEQ.ID.NO.:3). FIG. 19C shows the corresponding amino acid sequence (SEQ.ID.NO.:4). From GenBank accession no. AF073931.

FIG. 20A-B shows a nucleotide sequence encoding a splice variant of the α1B subunit of the human N-type calcium channel (SEQ.ID.NO.:5). Figure 20C shows the corresponding amino acid sequence (SEQ.ID.NO.:6). From GenBank accession no. M94172.

FIG. 21A-B shows a nucleotide sequence encoding a splice variant of the α1B subunit of the human N-type calcium channel (SEQ.ID.NO.:7). FIG. 21C shows the corresponding amino acid sequence (SEQ.ID.NO.:8). From GenBank accession no. M94173.

FIG. 22A-B shows a nucleotide sequence encoding the human calcium channel α1A isoform 1A-1 subunit (SEQ.ID.NO.:9). FIG. 22C shows the corresponding amino acid sequence (SEQ.ID.NO.:10). From GenBank accession no. AF004884.

FIG. 23A-B shows a nucleotide sequence encoding the human calcium channel α1A isoform 1A-2 subunit (SEQ.ID.NO.:11). FIG. 23C shows the corresponding amino acid sequence (SEQ.ID.NO.:12). From GenBank accession no. AF004883.

FIG. 24 shows a schematic diagram of one embodiment of a EFS system utilizing a computer, voltage generator, amplifier, membrane bottom wells, common trough, and fluorescence detector, inter alia.

FIG. 25 is a photograph showing an electrode head embodiment especially adapted for use with a 96 well tray.

FIG. 26 is a photograph showing a trough embodiment for use in conjunction with the electrode head embodiment shown in FIG. 25.

FIG. 27 is a photograph showing the trough embodiment of FIG. 26 with a multi-screen well tray positioned therein.

FIG. 28 is a photograph showing the assembled electrode head, trough and multiscreen.

FIG. 29 shows a graphical representation of data obtained from an embodiment of the invention similar to that depicted in FIG. 28. The data represent a membrane potential change in HEK293 cells that have been transfected to express human PN1 voltage-gated sodium channel. Each plot represents a row (12wells) A-H of a 96-well plate. Each column of the 96-well plate data was acquired for 15 seconds on a VIPR™. Stimulation pulse protocol was applied during the data acquisition as follows; 2 s baseline was followed with a 2 ms square pulse, Amplitude=20 mA, Frequency=10 Hz, Duration=5 s.

FIG. 30 is a bar graph representation of the peak ration change of data depicted in FIG. 29. 1 μM TTX a specific and potent blocker of tetrodotoxin (TTX)-sensitive voltage-gated sodium channels is present in wells E1, F1, G1, H1, A12, B12, C12 and D12. In addition well A11 contains an internal standard for blocking TTX-sensitive voltage-gated sodium channels. Z-score is a measure of the difference in the uninhibited and inhibited signal divided by the sum of the standard deviations.

FIG. 31 shows the effects of increasing concentrations of TTX (upper panel) and of Compound A (lower panel) on the EFS-stimulated depolarization signal in HEK293/PN1 cells. The IC₅₀s obtained in these experiments are comparable to those obtained through other techniques. The high Hill coefficients, nH, result from the threshold nature of the stimulation protocol.

FIG. 32 is a photograph showing an alternative embodiment. FIG. 32 shows an electrode head similar to that shown in FIG. 25, and a copper electrode plate. This embodiment is especially adapted for use with Caco-2 multiscreens (Millipore, Beford, Mass.).

FIG. 33 is a photograph similar to that shown in FIG. 32 except that the copper electrode plate has been turned over to show conducting pins (note: pins extend out of page toward reader).

FIG. 34 is a photograph showing the copper electrode plate placed on top of an assembled Caco-2 membrane bottom well and receiver tray.

FIG. 35 is a photograph showing the assembled embodiment of FIG. 34, i.e., electrode head, copper electrode plate with pins, Caco-2 membrane bottom well, and Caco-2 receiver tray.

FIG. 36 depicts a novel electrode embodiment that comprises a dielectric disc sandwiched between two conductive discs. FIG. 36A shows an expanded view of the novel electrode embodiment. FIG. 36B shows the novel electrode embodiment electrically connected to a concentric lead. FIG. 36C shows the novel electrode embodiment electrically connected to edge leads.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides equipment and techniques to implement electric field stimulation (EFS) of cells while monitoring a biological response of the cells. Preferably, the biological response is monitored by fluorescence detection. The cells are grown and/or attached to specially designed substrates such as, e.g., glass slides which contain preferably transparent, electrically conductive electrodes or multiwell tissue culture plates containing electrodes so disposed that when a preselected voltage is applied across the electrodes the transmembrane potential of cells within the wells of the multiwell tissue culture plates is altered.

In general terms, the present invention involves providing a substrate upon which living eukaryotic cells, preferably mammalian cells, are present where the cells express voltage-gated ion channels in their plasma membranes. Positive and negative electrodes are positioned either on or near the substrate so that when a voltage is applied through the electrodes the voltage-gated ion channels either open or close, thereby modulating the flow of at least one type of ion through the plasma membranes of the cells. This modulation of ion flow, or a change in membrane potential that results from the modulation of ion flow, is detected, either directly or indirectly, preferably by the use of fluorescent indicator compounds in the cells. Collections of substances, e.g., combinatorial libraries of small organic molecules, natural products, phage display peptide libraries, etc., are brought into contact with the voltage-gated ion channels in the plasma membranes of the cells and those substances that are able to affect the modulation of ion flow are identified. In this way, the present invention provides methods of screening for activators and inhibitors of voltage-gated ion channels. Such activators and inhibitors are expected to be useful as pharmaceuticals or as lead compounds from which pharmaceuticals can be developed by the usual processes of drug development, e.g., medicinal chemistry.

During an applied extracellular electrical field, the cell membrane electrical capacitance will charge or discharge depending upon the polarity and orientation of the cell relative to the field. This results in a transient change in the transmembrane potential in a given patch of membrane. These transient changes in transmembrane potential will vary continuously around each cell depending upon the orientation of each patch of membrane relative to the applied field and the existing transmembrane potential. In each membrane patch, membrane potential will be perturbed away from the resting value by the applied external field. This change in membrane potential will in turn affect the proportion of open and closed voltage-gated ion channels in each local patch of membrane, which will affect the conductance of the voltage-gated ion channels and thus change the membrane potential further. This process is expected to vary around each cell such that, in any given cell, different patches of membrane and the embedded voltage-gated ion channels will experience different membrane potentials. In general, the membrane potential in a given patch of membrane will change at a rate that is proportional to its resistance (1/conductance) and its capacitance (C_(m)) such that dV/dt=I/C_(m) where I is the total current flow (I=V/R) across the patch of membrane.

FIG. 14 illustrates these concepts. For the sake of simplicity, the plasma membrane of the cell shown in FIG. 14 is divided into four patches: left, top, right, and bottom. Current will flow between the electrodes if a voltage difference is applied. This will alter the cell membrane potential. If electrode 1 is positive and electrode 2 is negative, the membrane patch at the bottom of the cell will be hyperpolarized but the top patch will be depolarized. The left and right patches will “see” no change in membrane potential. If polarity is reversed, the opposite will occur.

In reality, of course, the cell's plasma membrane is a continuum of individual patches rather the simplified system of four patches depicted in FIG. 14. The applied voltage alters the membrane potentials of the various patches to many different values such that the embedded voltage-gated ion channels “sample” the many different potentials and are driven through their various conformational states. These include open states, closed states, high affinity drug bound states, and low affinity drug bound states.

Accordingly, the present invention provides a method for identifying modulators of the activity of a voltage-gated ion channel comprising:

-   -   (a) altering the transmembrane potential of at least a portion         of the membrane of a cell expressing the voltage-gated ion         channel by applying a voltage to the cells through extracellular         electrodes while monitoring ion flow through the voltage-gated         ion channel;     -   (b) exposing the cell in step (a) to a substance and monitoring         ion flow through the voltage-gated ion channel;     -   (c) comparing the ion flow through the voltage-gated ion channel         in step (a) and step (b);     -   where a difference in the ion flow through the voltage-gated ion         channel in step (a) and step (b) indicates that the substance is         a modulator of the voltage-gated ion channel.

A variation of the method comprises:

-   -   (a) dividing a plurality of cells expressing the voltage-gated         ion channel into a control portion and a test portion;     -   (b) altering the transmembrane potential of the control portion         of cells by applying a voltage to the cells through         extracellular electrodes while monitoring ion flow through the         voltage-gated ion channel;     -   (c) altering the transmembrane potential of the test portion of         cells by applying the voltage to the cells through extracellular         electrodes in the presence of a substance while monitoring ion         flow through the voltage-gated ion channel;     -   (d) comparing the ion flow through the voltage-gated ion channel         in step (b) and step (c);     -   where a difference in the ion flow through the voltage-gated ion         channel in step (b) and step (c) indicates that the substance is         modulator of the voltage-gated ion channel.

For the sake of simplicity, the above methods are described in terms of “a” voltage-gated ion channel although those skilled in the art will understand that in actual practice the cells will express a plurality of the voltage-gated ion channels for which modulators are sought. Generally, each cell will express at least 10², 10³, 10⁴, 10⁵, 10⁶ or more molecules of the voltage-gated ion channel. Also, ion flow will be monitored through the plurality of the voltage-gated ion channels rather than through a single voltage-gated ion channel. Similarly, the methods will generally be practiced by employing a plurality of cells, even though the methods are described above in terms of “a” cell.

Generally, the methods of the present invention will be carried out on a substrate that is a modified version of a standard multiwell tissue culture plate or microtiter plate. Such substrates will have a place for the cells to be tested (generally the wells of the tissue culture plate or microtiter plate) and will have positive and negative electrodes (either built into the plate or nearby) in such an orientations with respect to the cells that the electrodes can deliver a voltage potential that causes an alteration in the open/close state of the voltage-gated ion channels in the cells. The electrodes are extracellular, i.e., they do not penetrate into or across the plasma membranes of the cells although they may touch the outside of the plasma membranes in certain embodiments. Extracellular electrodes do not include electrodes which form a continuous connection with a cell's interior, e.g., patch/clamp electrodes.

Therefore, the present invention provides a method of identifying activators of a voltage-gated ion channel comprising:

-   -   (a) providing a substrate upon which are living eukaryotic cells         that express a plurality of the voltage-gated ion channels in         their plasma membranes;     -   (b) providing positive and negative electrodes positioned either         on or near the substrate such that when a preselected voltage is         applied through the positive and negative electrodes the         transmembrane electrical potential of the cells is altered such         that at least a portion of the voltage-gated ion channels are         closed;     -   (c) applying the preselected voltage through the positive and         negative electrodes;     -   (d) determining a control value for the flow of ions through the         voltage-gated ion channels of the cells in step (c);     -   (e) exposing the cells of step (c) to a substance for a period         sufficient and under conditions such that a detectable number of         the portion of the voltage-gated ion channels that are closed         become open and allow ion flow through the detectable number of         voltage-gated ion channels if the substance is an activator of         the voltage-gated ion channels;     -   (f) determining a test value for the flow of ions through the         voltage-gated ion channels of the cells of step (e);     -   (g) comparing the control value to the test value;     -   where if the control value is less than the test value, then the         substance is an activator of the voltage-gated ion channel.

The above-described method can be easily modified to provide a method for identifying inhibitors of the voltage-gated ion channel. The voltage applied through the electrodes is preselected such that it alters the electrical field around the cells and consequently alters the transmembrane electrical field. This in turn changes the states of the embedded voltage-gated ion channels such that instead of the voltage-gated ion channels being closed, the voltage-gated ion channels may open. Substances are then screened for the ability to close or inhibit the channels.

Accordingly, the present invention provides a method of identifying inhibitors of a voltage-gated ion channel comprising:

-   -   (a) providing a substrate upon which are living eukaryotic cells         that express a plurality of the voltage-gated ion channels in         their plasma membranes;     -   (b) providing positive and negative electrodes positioned either         on or near the substrate such that when a preselected voltage is         applied through the positive and negative electrodes the         transmembrane electrical potential of the cells is altered such         that at least a portion of the voltage-gated ion channels are         open;     -   (c) applying the preselected voltage through the positive and         negative electrodes;     -   (d) determining a control value for the flow of ions through the         voltage-gated ion channels of the cells in step (c);     -   (e) exposing the cells of step (c) to a substance for a period         sufficient and under conditions such that a detectable number of         the portion of the voltage-gated ion channels that are open         become closed and restrict ion flow through the detectable         number of voltage-gated ion channels if the substance is an         inhibitor of the voltage-gated ion channels;     -   (f) determining a test value for the flow of ions through the         voltage-gated ion channels of the cells of step (e);     -   (g) comparing the control value to the test value;     -   where if the control value is greater than the test value, then         the substance is an inhibitor of the voltage-gated ion channel.

In the above-described method for identifying activators, the terms “a portion of the voltage-gated ion channels are closed” and “a detectable number” are related and have relative rather than absolute values. Similarly, in the above- described method for identifying inhibitors, the terms “a portion of the voltage-gated ion channels are open” and “a detectable number” are also related and have relative rather than absolute values. What is meant is that a portion of the voltage-gated ion channels will be open or closed such that when the substance acts on the channels, a change in the open/closed state of a sufficient number of channels (i.e., “a detectable number”) occurs such that a difference in ion flow that is large enough to be measured by the detection system employed takes place. There is no need to determine the actual number of ion channels that constitutes the “portion” of voltage-gated ion channels that are closed or open or the “detectable number” so long as the difference in ion flow can be measured. The actual portion of channels that will be open or closed as well as the actual value of “detectable number” in order for the methods to be practiced will depend on such variables as the channel that is being studied, the concentrations of the substances tested, the nature of the detection system for ion flow, and so forth. Adjusting the voltage applied through the electrodes to take into account such variables so that control and test values can be obtained is a matter of routine experimentation in which the skilled artisan will be guided by knowledge in the art such as, e.g., the known voltage dependence of the open/close transition of the voltage-gated ion channel under study, the nature and sensitivity of the detection system employed to monitor the flow of ions, the level of expression of the ion channel in the cells, and so forth.

The electrodes can be arranged in a variety of ways in order to provide for the proper stimulus. A number of arrangements are described herein and illustrated in the accompanying figures. These include arrangements where the cells are present in wells in the substrate and:

-   -   (a) both a positive and negative electrode is present in each         well;     -   (b) one electrode is present in the well and the other electrode         enters the fluid medium in the well from above without touching         the sides or bottom of the well;     -   (c) the electrodes form part of the sides or bottom of the         wells;     -   (d) a pattern of interdigitating electrodes has been formed on         the surface of the substrate and at least some of the cells are         positioned between the interdigitating branches of the positive         and negative electrodes.

The skilled person will recognize that it is generally beneficial to run controls together with the methods described herein. For example, it will usually be helpful to have a control in which the substances are tested in the methods against cells that preferably are essentially identical to the cells that are used in the methods except that these cells would not express the voltage-gated ion channels of interest. In this way it can be determined that substances which are identified by the methods are really exerting their effects through the voltage-gated ion channels of interest rather than through some unexpected non-specific mechanism. One possibility for such control cells would be to use non-recombinant parent cells where the cells of the actual experiment express the voltage-gated ion channels of interest due to the recombinant expression of those voltage-gated ion channels of interest.

Other types of controls would involve taking substances that are identified by the methods of the present invention as activators or inhibitors of voltage-gated ion channels of interest and testing those substances in the methods of the prior art in order to confirm that those substances are also activators and inhibitors when tested in those prior art methods.

One skilled in the art would recognize that, where the present invention involves comparing control values for the flow of ions to test values for the flow of ions and determining whether the control values are greater or less than the test values, a non-trivial difference is sought. For example, if in the methods of identifying inhibitors, the control value were found to be 1% greater than the test value, this would not indicate that the substance is an inhibitor. Rather, one skilled in the art would attribute such a small difference to normal experimental variance. What is looked for is a significant difference between control and test values. For the purposes of this invention, a significant difference fulfills the usual requirements for a statistically valid measurement of a biological signal. For example, depending upon the details of the experimental arrangement, a significant difference might be a difference of at least 10%, preferably at least 20%, more preferably at least 50%, and most preferably at least 100%.

One skilled in the art would understand that the cells that give rise to the control values need not be physically the same cells that give rise to the test values, although that is possible. What is necessary is that the cells that give rise to the control values be substantially the same type of cell as the cells that give rise to the test values. A cell line that has been transfected with and expresses a certain voltage-gated ion channel could be used for both the control and test cells. Large numbers of such cells could be grown and a portion of those cells could be exposed to the substance and thus serve as the cells giving rise to the test value for ion flow while a portion would not be exposed to the substance and would thus serve as the cells giving rise to the control value for ion flow. No individual cell itself would be both control and test cell but the virtual identity of all the cells in the cell line ensures that the methods would nevertheless be reliable.

Accordingly, the present invention provides a method of identifying activators of a voltage-gated ion channel comprising:

-   -   (a) providing a substrate upon which are living eukaryotic cells         that express a plurality of the voltage-gated ion channels in         their plasma membranes;     -   (b) providing positive and negative electrodes positioned either         on or near the substrate such that when a preselected voltage is         applied through the positive and negative electrodes the         transmembrane electrical potential of the cells is altered such         that at least a portion of the voltage-gated ion channels are         closed;     -   (c) applying the preselected voltage through the positive and         negative electrodes to a control sample of the cells;     -   (d) determining a control value for the flow of ions through the         voltage-gated ion channels of the control sample of the cells in         step (c);     -   (e) applying the preselected voltage through the positive and         negative electrodes to a test sample of the cells while exposing         the test sample of the cells to a substance for a period         sufficient and under conditions such that a detectable number of         the portion of the voltage-gated ion channels that are closed in         the test sample become open and allow ion flow through the         detectable number of voltage- gated ion channels if the         substance is an activator of the voltage-gated ion channels;     -   (f) determining a test value for the flow of ions through the         voltage-gated ion channels of the test sample of cells of step         (e);     -   (g) comparing the control value to the test value;     -   where if the control value is less than the test value, then the         substance is an activator of the voltage-gated ion channel.

Similarly, the present invention provides a method of identifying inhibitors of a voltage-gated ion channel comprising:

-   -   (a) providing a substrate upon which are living eukaryotic cells         that express a plurality of the voltage-gated ion channels in         their plasma membranes;     -   (b) providing positive and negative electrodes positioned either         on or near the substrate such that when a preselected voltage is         applied through the positive and negative electrodes the         transmembrane electrical potential of the cells is altered such         that at least a portion of the voltage-gated ion channels are         open;     -   (c) applying the preselected voltage through the positive and         negative electrodes to a control sample of the cells;     -   (d) determining a control value for the flow of ions through the         voltage-gated ion channels of the control sample of the cells in         step (c);     -   (e) applying the preselected voltage through the positive and         negative electrodes to a test sample of the cells while exposing         the test sample of the cells to a substance for a period         sufficient and under conditions such that a detectable number of         the portion of the voltage-gated ion channels that are open in         the test sample become closed and restrict ion flow through the         detectable number of voltage- gated ion channels if the         substance is an inhibitor of the voltage-gated ion channels;     -   (f) determining a test value for the flow of ions through the         voltage-gated ion channels of the test sample of cells of step         (e);     -   (g) comparing the control value to the test value;     -   where if the control value is greater than the test value, then         the substance is an inhibitor of the voltage-gated ion channel.

“Substances” can be any substances that are generally screened in the pharmaceutical industry during the drug development process. For example, substances may be low molecular weight organic compounds (e.g., having a molecular weight of less than about 1,000 daltons); RNA, DNA, antibodies, peptides, or proteins.

The conditions under which cells are exposed to substances in the methods described herein are conditions that are typically used in the art for the study of protein-ligand interactions: e.g., physiological pH; salt conditions such as those represented by such commonly used buffers as PBS or in tissue culture media; a temperature of about 4° C. to about 55° C.; incubation times of from several seconds to several hours. Generally, the cells are present in wells in the substrate and the substances are added directly to the wells, optionally after first washing away the media in the wells.

Determining the values of ion flow in the methods of the present invention can be accomplished through the use of fluorescent indicator compounds.

One type of fluorescent indicator compound is sensitive to the level of intracellular calcium ions in the cells used in the present invention. This type of fluorescent indicator compound can be used when the methods are directed to those voltage-gated ion channels whose activity results in a change in intracellular calcium levels. Such voltage-gated ion channels include not only voltage-gated calcium channels but also other types of voltage-gated ion channels where the activity of those channels is naturally or can be coupled to changes in intracellular calcium levels. Many types of voltage-gated potassium channels can be so coupled. When using this approach to study a voltage-gated ion channel of interest that is not a voltage-gated calcium channel, it may be desirable to engineer the cells employed so as to recombinantly express voltage-gated calcium channels that are coupled to the voltage-gated ion channel of interest.

Fluorescent indicator compounds suitable for measuring intracellular calcium levels include various calcium indicator dyes (e.g., fura-2, fluo-3, indo-1, Calcium Green; see Veliçelebi et al., 1999, Meth. Enzymol. 294:2047).

Calcium indicator dyes are substances which show a change in a fluorescent characteristic upon binding calcium, e.g., greatly increased intensity of fluorescence and/or a change in fluorescent spectra (i.e., a change in emission or excitation maxima). Fluo-3, fura-2, and indo-1 are commonly used calcium indicator dyes that were designed as structural analogs of the highly selective calcium chelators ethylene glycol-bis(β-aminoethyl ether) N,N,N′,N′-tetraacetic acid (EGTA) and 1,2- bis(2-aminophenoxy) ethane-N,N,N′,N′-tetraacetic acid (BAPTA). The fluorescence intensity from fluo-3 increases by more than 100-fold upon binding of calcium. While the unbound dye exhibits very little fluorescence, calcium-bound fluo-3 shows strong fluorescence emission at 526 nm. Fura-2 is an example of a dye that exhibits a change in its fluorescence spectrum upon calcium binding. In the unbound state, fura- 2 has an excitation maximum of 362 nm. This excitation maximum shifts to 335 nm upon calcium binding, although there is no change in emission maximum. Binding of calcium to fura-2 can be monitored by excitation at the two excitation maxima and determining the ratio of the amount of fluorescence emission following excitation at 362 nm compared to the amount of fluorescence emission following excitation at 335 nm. A smaller ratio (i.e., less emission following excitation at 362 nm) indicates that more fura-2 is bound to calcium, and thus a higher internal calcium concentration in the cell.

The use of calcium indicator dyes entails loading cells with the dye, a process which can be accomplished by exposing cells to the membrane-permeable acetoxymethyl esters of the dyes. Once inside the plasma membrane of the cells, intracellular esterases cleave the esters, exposing negative charges in the free dyes. This prevents the free dyes from crossing the plasma membrane and thus leaves the free dyes trapped in the cells. Measurements of fluorescence from the dyes are then made, the cells are treated in such a way that the internal calcium concentration is changed (e.g., by exposing cells to an activator or inhibitor of a voltage-gated ion channel), and fluorescence measurements are again taken.

Fluorescence from the indicator dyes can be measured with a luminometer or a fluorescence imager. One preferred detection instrument is the Fluorometric Imaging Plate Reader (FLIPR) (Molecular Devices, Sunnyvale, Calif.). The FLIPR is well suited to high throughput screening using the methods of the present invention as it incorporates integrated liquid handling capable of simultaneously pipetting to 96 or 384 wells of a microtiter plate and rapid kinetic detection using a argon laser coupled to a charge-coupled device imaging camera.

A typical protocol for use of calcium indicator dyes would entail plating cells expressing a voltage-gated ion channel of interest into an appropriate substrate (e.g., clear, flat-bottom, black-wall 96 well plates that have a suitable arrangement of positive and negative electrodes) and allowing the cells to grow overnight in standard tissue culture conditions (e.g., 5% CO₂, 37° C.). The cells are generally plated at a density of about 10,000 to 100,000 cells per well in appropriate growth medium. On the day of the assay, growth medium is removed and dye loading medium is added to the wells.

If the calcium indicator dye is fluo-3, e.g., dye loading medium could be prepared by solubilizing 50 μg of fluo-3-AM ester (Molecular Probes F-1242) in 22 μl DMSO to give a 2 mM dye stock. Immediately before loading the cells, 22 μl 20% pluronic acid (Molecular Probes P-3000) is added to the dye. The tube containing the dye is mixed with a vortex mixer and 42 ml of the dye/pluronic acid solution is added to 10.5 ml of Hanks Balanced Salt Solution (Gibco/BRL Cat # 14025-076) with 20 mM HEPES (Gibco/BRL Cat # 1560-080), 2.5 mM probenecid (Sigma Cat # P-8761), and 1% fetal bovine serum (Gibco/BRL Cat # 26140-087; not BSA)). The dye and the loading medium are mixed by repeated inversion (final dye concentration about 4 μM).

Growth medium can be removed from the cells by washing (wash medium is Hanks Balanced Salt Solution (Gibco/BRL Cat # 14025-076) with 20 mM HEPES (Gibco/BRL Cat # 1560-080), 2.5 mM probenecid (Sigma Cat # P-8761), and 0.1% bovine serum albumin (Sigma Cat # A-9647; not FBS) three times, leaving 100 μl residual medium in the wells after the fourth wash. Then 100 μl of the dye in the loading medium is added to each well. The cells are then incubated for 60 minutes to allow for dye loading.

Following dye loading, fluorescent measurements of the cells are taken prior to exposure of the cells to substances that are to be tested. The cells are then exposed to the substances and those substances that cause a change in a fluorescent characteristic of the dye are identified. The measuring instrument can be a fluorescent plate reader such as the FLIPR (Molecular Devices). Substances that cause a change in a fluorescent characteristic in the test cells but not the control cells are possible activators or inhibitors of the voltage-gated ion channel.

The exact details of the procedure outlined above are meant to be illustrative. One skilled in the art would be able to optimize experimental parameters (cell number, dye concentration, dye loading time, temperature of incubations, cell washing conditions, and instrument settings, etc.) by routine experimentation depending on the particular relevant experimental variables (e.g., type of cell used, identity of dye used). Several examples of experimental protocols that can be used are described in Veliçelebi et al., 1999, Meth. Enzymol. 294:2047. Other suitable instrumentation and methods for measuring transmembrane potential changes via optical methods includes microscopes, multiwell plate readers and other instrumentation that is capable of rapid, sensitive ratiometric fluorescence detection. For example, the VIPR (Aurora Biosciences, San Diego, Calif.) is an integrated liquid handler and kinetic fluorescence reader for 96-well and greater multiwell plates. The VIPR reader integrates an eight channel liquid handler, a multiwell positioning stage and a fiber-optic illumination and detection system. The system is designed to measure fluorescence from a column of eight wells simultaneously before, during and after the introduction of liquid sample obtained from another microtiter plate or trough. The VIPR reader excites and detects emission signals from the bottom of a multiwell plate by employing eight trifurcated optical bundles (one bundle for each well). One leg of the trifurcated fiber is used as an excitation source, the other two legs of the trifurcated fiber being used to detect fluorescence emission. A ball lens on the end of the fiber increases the efficiency of light excitation and collection. The bifurcated emission fibers allow the reader to detect two emission signals simultaneously and are compatible with rapid signals generated by the FRET-based voltage dyes.

Photomultiplier tubes then detect emission fluorescence, enabling sub-second emission ratio detection.

In particular embodiments, the calcium indicator dye is selected from the group consisting of: fluo-3, fura-2, fluo-4, fluo-5, calcium green-1, Oregon green, 488 BAPTA, SNARF-1, and indo-1.

In particular embodiments, the change in fluorescent characteristic is an increase in intensity of a fluorescence emission maximum. In other embodiments, the change in fluorescent characteristic is a shift in the wavelength of an absorption maximum.

In particular embodiments, the cells naturally express the voltage-gated ion channel of interest and/or calcium channels. In other embodiments, the cells do not naturally express the voltage-gated ion channel of interest and/or calcium channels but instead have been transfected with expression vectors that encode the voltage-gated ion channel of interest and/or calcium channels so that the cells recombinantly express the voltage-gated ion channel of interest and/or calcium channels. Transfection is meant to include any method known in the art for introducing expression vectors into the cells. For example, transfection includes calcium phosphate or calcium chloride mediated transfection, lipofection, infection with a retroviral construct, and electroporation.

An alternative to the use of calcium indicator dyes is the use of the aequorin system. The aequorin system makes use of the protein apoaequorin, which binds to the lipophilic chromophore coelenterazine forming a combination of apoaequorin and coelenterazine that is known as aequorin. Apoaequorin has three calcium binding sites and, upon calcium binding, the apoaequorin portion of aequorin changes its conformation. This change in conformation causes coelenterazine to be oxidized into coelenteramide, CO₂, and a photon of blue light (466 nm). This photon can be detected with suitable instrumentation.

Since the gene encoding apoaequorin has been cloned (U.S. Pat. No. 5,541,309; U.S. Pat. No. 5,422,266; U.S. Pat. No. 5,744,579; Inouye et al., 1985, Proc. Natl. Acad. Sci. USA 82:3154-3158; Prasher et al., 1985, Biochem. Biophys. Res. Comm. 126:1259-1268), apoaequorin can be recombinantly expressed in cells in which it is desired to measure the intracellular calcium concentration. Alternatively, existing cells that stably express recombinant apoaequorin can be used. Such cells derived from HEK293 cells and CHO-K1 cells are described in Button & Brownstein, 1993, Cell Calcium 14:663-671. For example, the HEK293/aeq17 cell line can be used as follows.

The HEK293/aeq17 cells are grown in Dulbecco's Modified Medium (DMEM, GIBCO-BRL, Gaithersburg, Md., USA) with 10% fetal bovine serum (heat inactivated), 1 mM sodium pyruvate, 500 μg/ml Geneticin, 100 μg/ml streptomycin, 100 units/ml penicillin. Expression vectors encoding the voltage-gated ion channel of interest as well as, optionally, the desired voltage-gated calcium channel subunits (α_(1A), α_(1B), α_(1C), α_(1D), α_(1E), α_(1G), α_(1H), α_(1I), α₂δ, β₁, β₂, β₃, β₄, etc.) can be transfected into the HEK293/aeq17 cells by standard methods in order to express the desired voltage-gated ion channel subunits and voltage-gated calcium channel subunits in the HEK293/aeq17 cells. The cells are washed once with DMEM plus 0.1 % fetal bovine serum, and then charged for one hour at 37° C./5% CO₂ in DMEM containing 8 μM coelenterazine cp (Molecular Probes, Eugene, Oreg., USA) and 30 μM glutathione. The cells are then washed once with Versene (GIBCO-BRL, Gaithersburg, Md., USA), detached using Enzyme-free cellissociation buffer (GIBCO-BRL, Gaithersburg, Md., USA), diluted into ECB (Ham's F12 nutrient mixture (GIBCO-BRL) with 0.3 mM CaCl₂, 25 mM HEPES, pH7.3, 0.1% fetal bovine serum). The cell suspension is centrifuged at 500×g for 5 min. The supernatant is removed, and the pellet is resuspended in 10 ml ECB. The cell density is determined by counting with a hemacytometer and adjusted to 500,000 cells/ml in ECB. The substances to be tested are diluted to the desired concentrations in ECB and aliquoted into the assay plates, preferably in triplicate, at 0.1 ml/well. The cell suspension is injected at 0.1 ml/well, read and integrated for a total of 400 readings using a luminometer (Luminoskan Ascent, Labsystems Oy, Helsinki, Finland).

Alternatively, the cells may first be placed into the assay plates and then the substances added. Data are analyzed using the software GraphPad Prism Version 3.0 (GraphPad Software, Inc., San Diego, Calif., USA).

It will be understood by those skilled in the art that the procedure outlined above is a general guide in which the various steps and variables can be modified somewhat to take into account the specific details of the particular assay that is desired to be run. For example, one could use semisynthetic coelenterazine (Shimomura, 1989, Biochem. J. 261:913-920; Shimomura et al., 1993, Cell Calcium 14:373-378); the time of incubation of the cells with coelenterazine can be varied somewhat; somewhat greater or lesser numbers of cells per well can be used; and so forth.

For reviews on the use of aequorin, see Creton et al., 1999, Microscopy Research and Technique 46:390-397; Brini et al., 1995, J. Biol. Chem. 270:9896- 9903; Knight & Knight, 1995, Meth. Cell. Biol. 49:201-216. Also of interest may be U.S. Pat. No. 5,714,666 which describes methods of measuring intracellular calcium in mammalian cells by the addition of coelenterazine co-factors to mammalian cells that express apoaequorin.

Another way to measure ion flow is to monitor changes in transcription that result from the activity of voltage-gated ion channels by the use of transcription based assays. Transcription-based assays involve the use of a reporter gene whose transcription is driven by an inducible promoter whose activity is regulated by a particular intracellular event such as, e.g., changes in intracellular calcium levels, that are caused by the activity of a voltage-gated ion channel. Transcription-based assays are reviewed in Rutter et al., 1998, Chemistry & Biology 5:R285-R290. Transcription-based assays of the present invention rely on the expression of reporter genes whose transcription is activated or repressed as a result of intracellular events that are caused by the interaction of a activator or inhibitor with a voltage-gated ion channel.

An extremely sensitive transcription-based assay is disclosed in Zlokarnik et al., 1998, Science 279: 84-88 (Zlokarnik) and also in U.S. Pat. No. 5,741,657. The assay disclosed in Zlokarnik and U.S. Pat. No. 5,741,657 employs a plasmid encoding β-lactamase under the control of an inducible promoter. This plasmid is transfected into cells together with a plasmid encoding a receptor for which it is desired to identify agonists. The inducible promoter on the β-lactamase is chosen so that it responds to at least one intracellular signal that is generated when an agonist binds to the receptor. Thus, following such binding of agonist to receptor, the level of β-lactamase in the transfected cells increases. This increase in β-lactamase is measured by treating the cells with a cell-permeable dye that is a substrate for cleavage by β-lactamase. The dye contains two fluorescent moieties. In the intact dye, the two fluorescent moieties are physically linked, and thus close enough to one another that fluorescence resonance energy transfer (FRET) can take place between them. Following cleavage of the dye into two parts by β-lactamase, the two fluorescent moieties are located on different parts, and thus can diffuse apart. This increases the distance between the fluorescent moieties, thus decreasing the amount of FRET that can occur between them. It is this decrease in FRET that is measured in the assay.

The assay described in Zlokarnik and U.S. Pat. No. 5,741,657 can be modified for use in the methods of the present invention by using an inducible promoter to drive β-lactamase where the promoter is activated by an intracellular signal generated by the opening or closing of a voltage-gated ion channel. Cells expressing a voltage-gated ion channel and the inducible promoter-driven β-lactamase are placed in the apparatus of the present invention, where the open or closed state of the voltage-gated ion channels can be controlled. The cells are exposed to the cell- permeable dye and then exposed to substances suspected of being activators or inhibitors of the voltage-gated ion channel. Those substances that cause a change in the open or closed state of the voltage-gated ion channel are identified by their effect on the inducible promoter-driven β-lactamase and thus on FRIT. The inducible promoter-driven β-lactamase is engineered with a suitable promoter so that β-lactamase is induced when the substance is either an activator or an inhibitor, depending upon the nature of the assay.

The flow of ions through voltage-gated ion channels can also be measured by measuring changes in membrane potential via the use of fluorescent voltage sensitive dyes. The changes in membrane potential will depend on the ion channels in the cell membrane. The resultant membrane potential will depend on the net properties of all the channels and the change caused by inhibiting (through a substance that is an inhibitor or antagonist) or activating (through a substance that is an activator or an agonist) the voltage-gated ion channel of interest. One knowledgeable in cellular and membrane biophysics and electrophysiology will understand the directions of the changes in membrane potential since those changes depend on the ion channels present and the inhibition or activation of those channels by test substances. In many cases when using fluorescent voltage sensitive dyes, the experimental system can be calibrated by using known activators or inhibitors of the voltage-gated ion channel of interest.

The present invention therefore includes assays that monitor changes in ion flow caused by activators or inhibitors of voltage-gated ion channels based upon FRET between a first and a second fluorescent dye where the first dye is bound to one side of the plasma membrane of a cell expressing a voltage-gated ion channel of interest and the second dye is free to move from one face of the membrane to the other face in response to changes in membrane potential. In certain embodiments, the first dye is impenetrable to the plasma membrane of the cells and is bound predominately to the extracellular surface of the plasma membrane. The second dye is trapped within the plasma membrane but is free to diffuse within the membrane. At normal (i.e., negative) resting potentials of the membrane, the second dye is bound predominately to the inner surface of the extracellular face of the plasma membrane, thus placing the second dye in close proximity to the first dye. This close proximity allows for the generation of a large amount of FRET between the two dyes. Following membrane depolarization, the second dye moves from the extracellular face of the membrane to the intracellular face, thus increasing the distance between the dyes. This increased distance results in a decrease in RET, with a corresponding increase in fluorescent emission derived from the first dye and a corresponding decrease in the fluorescent emission from the second dye. See figure 1 of González & Tsien, 1997, Chemistry & Biology 4:269-277. See also González & Tsien, 1995, Biophys. J. 69:1272-1280 and U.S. Pat. No. 5,661,035.

In certain embodiments, the first dye is a fluorescent lectin or a fluorescent phospholipid that acts as the fluorescent donor. Examples of such a first dye are: a coumarin-labeled phosphatidylethanolamine (e.g., N-(6-chloro-7-hydroxy- 2-oxo-2H-1-benzopyran-3-carboxamidoacetyl)-dimyristoylphosphatidyl- ethanolamine) or N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)- dipalmitoylphosphatidylethanolamine); a fluorescently-labeled lectin (e.g., fluorescein-labeled wheat germ agglutinin). In certain embodiments, the second dye is an oxonol that acts as the fluorescent acceptor. Examples of such a second dye are: bis(1,3-dialkyl-2-thiobarbiturate)trimethineoxonols (e.g., bis(1,3-dihexyl-2 thiobarbiturate)trimethineoxonol) or pentamethineoxonol analogues (e.g., bis(1,3- dihexyl-2-thiobarbiturate)pentamethineoxonol; or bis(1,3-dibutyl-2- thiobarbiturate)pentamethineoxonol). See González & Tsien, 1997, Chemistry & Biology 4:269-277 for methods of synthesizing various dyes suitable for use in the present invention. In certain embodiments, the assay may comprise a natural carotenoid, e.g., astaxanthin, in order to reduce photodynamic damage due to singlet oxygen.

The use of such fluorescent dyes capable of moving from one face of the plasma membrane to the other is especially appropriate when the methods of the present invention are directed to inwardly rectifying potassium channels. Activation of inwardly rectifying potassium channels results in increased potassium current flow across the plasma membrane. This increased current flow results in a hyperpolarization of the cell membrane that can be detected by use of the technique described above since such hyperpolarization will result in greater FRET.

A large number of possible combinations of types of substrates and electrodes; physical arrangement of electrodes; number, shape, and arrangement of wells for holding the cells are suitable for use in the present invention.

FIG. 1 illustrates an embodiment of the invention where the electrodes are generally parallel wires or strips of conductive material such as gold. The electrodes lie on the surface of a glass substrate and, together with the spacers, form the walls of the wells. For clarity, only a single series of wells is shown in FIG. 1. Generally, substantially the entire surface of the glass substrate would be covered by wells formed in the manner shown. Cells are placed in the wells and grown in suitable media until an appropriate number of cells is present in the wells. Alternatively, an appropriate number of cells may be placed into the wells and used without further growth.

FIG. 2B illustrates an embodiment of the invention where the wells are cavities or depressions in the surface of the substrate, as in typical multiwell tissue culture plates. Each well has an electrode at the bottom of the well and another electrode that is aligned along a side of the well. The cells are shown in FIG. 2B as attached at the bottom of the well but in certain embodiments the cells may be suspension cells dispersed in the fluid in the well.

FIG. 2C illustrates an embodiment of the invention similar to that shown in FIG. 2B except that in FIG. 2C both electrodes are at the bottom of the wells.

FIG. 3 illustrates an embodiment of the invention where an array of interdigitating transparent electrodes has been chemically etched onto the surface of a glass substrate. The electrode array, comprising a comb of positive and negative electrodes, has been chemically etched onto an indium tin oxide (ITO) coated glass plate. The thin layer of ITO (about 200 Å to 2,000 Å, or 500 Å to 1,500 Å, preferably 1,200 Å thick) forms a transparent conductive coating on the surface of the glass. Although not essential, it is preferred that the layer of ITO be thin enough to be transparent. The chemical etching process removes the ITO from selected areas, resulting in an array of transparent ITO electrodes bonded to the glass. Multiple reaction wells may be contained on a single glass plate by forming fluid retention wells at the different electrode array sites. The wells can be formed by attaching (e.g., gluing) a well frame to the glass substrate or by forming virtual wells on the glass plate by a method such as screening hydrophobic ink onto the plate.

FIGS. 4A and 4B illustrates an embodiment in which wells are formed by attaching a well frame onto the substrate.

FIG. 6 illustrates an embodiment in which a droplet of fluid containing cells that express a voltage-gated ion channel is sandwiched between two plates. The plates, which can be glass plates, are each coated with a thin layer of conductive material such as indium tin oxide (ITO). The layers of conductive material are connected to a pulse generator such that one layer functions as a positive electrode and the other layer functions as a negative electrode.

FIGS. 7 and 8 illustrate embodiments in which one of the electrodes enters the well from above. In FIGS. 9 and 10, both electrodes enter from above.

The substrates for use in the present invention may contain virtual wells. Virtual wells are formed when a surface is patterned to have relatively hydrophilic domains within relatively hydrophobic fields so that an aqueous sample is physically constrained by surface tension to the more hydrophilic domains by the edges of the more hydrophobic fields. The hydrophilic domains can be small circles that form a pattern similar to the wells of a conventional microtiter plate. Virtual wells provide a location in which samples can be confined without the deep indentations found in conventional microtiter plates. FIG. 5 illustrates a surface for use in the present invention that is a derivatized glass surface upon which virtual wells have been formed and upon which a pattern of interdigitated electrodes has also been formed. FIG. 3 shows an individual well from this surface. International Patent Publication WO 99/39829 describes virtual wells and how they can be made.

“Interdigitating” refers to an arrangement of positive and negative electrodes where the positive and negative electrodes contain branches that are arranged such that, if a line were drawn from one branch of a positive electrode to the adjacent branch of the positive electrode, the line would cross a branch of the negative electrode. Similarly, if a line were drawn from one branch of a negative electrode to the adjacent branch of the negative electrode, the line would cross a branch of the positive electrode. Generally, each interdigitating positive or negative electrode has at least 2, or at least 4, or at least 10, or at least 20 interdigitating branches. An example of interdigitating electrodes is shown in FIG. 3.

Various additional arrangements of electrodes formed from conductive materials on glass substrates are possible. One arrangement has the positive and negative electrodes formed on two parallel glass substrates. For example, instead of having the positive and negative electrodes on a single glass substrate, two ITO coated glass substrates can be utilized by placing the glass substrates parallel to one another and placing the biologic fluid containing the cells in the gap between the glass substrates. In this arrangement, one conductive glass substrate serves as the positive electrode while the second glass substrate serves as the negative electrode. The electrode field is formed at a right angle to the surface of the plates. This arrangement would allow fluid containing the cells to be either dispensed in between the plates or drawn into the gap via capillary action. The detector's light beam would enter perpendicular to the glass substrates and pass into the gap between the glass substrates, illuminating the fluid and cells. The fluorescence transmission from the cells would be collected by the detector in a similar manner. FIG. 6 illustrates one version of this arrangement. Another version is shown in FIG. 13 where an embodiment comprising two ITO-coated plates each containing multiple virtual wells is depicted. The ITO forms the bottom of the wells as well as the electrodes.

Another arrangement has the positive and negative electrodes formed by a single glass substrate and a reference electrode. This arrangement utilizes a single glass substrate coated with a conductive material such as ITO as one electrode. A well holding the biological fluid and cells is formed on the surface of the conductive material coating the glass substrate. A wire or similar conducting member placed into the well serves as the second electrode. FIG. 7 illustrates a single well of a version of this arrangement. FIG. 12 depicts this type of arrangement as it is usually practiced, in a multiwell format. FIG. 15 shows a modification of this arrangement where one electrode is a highly conductive metal grid that is in contact with the ITO layer.

Another arrangement has the single conductive glass substrate acting as the conductor to the current generated by a positive and negative electrode pair placed in adjacent wells. See FIG. 16A-D. This arrangement does not use a grounding grid. The current flows from a first electrode in a first well through the ITO bottom of the first well to the ITO bottom of an adjacent second well and through a second electrode in the second well. Adjacent electrodes are alternately positive and negative. See FIGS. 16A and 16C.

In certain embodiments using interdigitating electrodes, the spacing and width of the branches of the electrodes are on the same order of magnitude as the size of individual cells. Cells may be grown and attached to the substrate in such a manner that, if a cell attaches between a pair of positive and negative electrode branches, a lower applied stimulus pulse can be utilized. The advantage of this close electrode spacing is that it results in less shunting of the stimulus current pulse through the fluid medium and less fluid heating while stimulating the cells. The use of transparent interdigitating electrodes offers the advantage of passing light from a fluorescent emission light source through the preferably glass substrate and transparent electrodes onto the cell and light passage of the fluorescence signal back to the light detector. While making the electrodes from a transparent material such as indium tin oxide (ITO) has advantages in certain embodiments, the electrodes may also be made from non-transparent conductive materials such as platinum, silver, or gold. If the electrode material is not transparent, fluorescence measurements are still possible because light can pass through the glass in between the electrodes.

Regardless of the arrangement of electrodes, stimulus pulses are generated by a pulse generator and applied to either a single well electrode array or to multiple well electrode arrays. Various commercial pulse generators can be utilized that permit waveform generation and amplitude adjustment. Constant voltage or constant current waveforms can be applied to the electrodes. Commercially available power supplies that can be used in the present invention include the STG 1004 or STG 1008 Stimulus Generator or the National Instruments PCI 6713 8 channel pcb.

In using the pulse generator to stimulate the cells, particular attention should be paid to the amplitude, pulse width, and polarity used. For certain extreme field strengths, electroporation of the biological membrane can occur, and this should be avoided. When changing the external electrical field, the desired goal is a change in the trans-membrane field (Vm) by less than approximately ±100 mV. As such the amount of charge added or removed from the cell membrane capacitance is critical. Adjustment of the pulse amplitude and duration is necessary to ensure a change in Vm without electroporation of the cells. Typically the voltage changes across the electrodes may be on the order of ±10 volts, preferably less than ±5 volts, and if possible less than ±1 volt. These values can be adjusted empirically, by routine experimentation, in order to optimize the cellular membrane potential change without electroporation of the cell membrane. In general, the amount of charge change on the cell membrane will depend upon the local field changes, which depend upon the electrical current. Adjusting the area (the current-time integral) of the applied current as determined by the change in external electric field can be readily optimized empirically. In general, if the goal is to stimulate a cellular action potential, the pulse duration will be kept brief and the amplitude will be increased up to a point that exceeds the threshold for action potential generation. This will be affected by the relative levels of ion channels expressed in the cells and will vary accordingly, requiring empirical adjustment. A typical value might be a pulse duration of 1 millisecond and a pulse amplitude of 5 volts; this might be varied to increase the duration to 2 milliseconds and decrease the amplitude to 2.5 volts, or to decrease the duration and increase the amplitude, etc. In general, there is an inverse parabolic relationship between the duration and the amplitude of the applied pulse, where the area of the applied current-time integral remains constant. Because ion channel kinetics and action potentials can be rapid and brief, minimizing the pulse duration is useful. These parameters will also depend upon the manufactured electrodes, their capacitance and resistance, the geometrical relationship to the cells, the ionic strength and composition of the solutions used, and the electrical coupling to the cells. Because of these many variables, an empirical approach based upon the above guidelines is best.

Electrode arrangements can be adapted to 12-well, 24-well, 96-well, 384-well, 1,536-well, 3,456-well, and other plate formats, permitting the present invention to be used in high throughput screening applications.

In embodiments of the invention such as that illustrated in FIG. 12 where multiple wells are present in the substrate and each well has an electrode associated with it, the stimulus delivered to each well through the electrodes can be individually controlled by the application of suitable software that governs the pulse generator. Such software is well known in the art or can be readily designed by one skilled in the art.

Particular embodiments of the present invention employ an arrangement of electrodes and wells on a substrate such that the substrate has the same form factor as a typical multiwell tissue culture plate that is used for high throughput screening, e.g., a 96 well plate. The spacing of the wells on the substrate can be such that the center-to-center distances of the wells on the substrate is the same as the typical center-to-center distances between wells on typical 96 well plates that are used for high throughput screening. This facilitates the use of the present invention with current equipment used in high throughput screening such as plate handlers, detectors, automatic pipettors, etc. Substrates can be manufactured by modifying the well-known manufacturing processes generally used to make multiwell tissue culture plates by adding electrodes to the plates according to one of the configurations of electrodes disclosed herein.

In particular embodiments of the present invention, the substrate is not silicon or a field effect transistor.

In particular embodiments of the present invention, cells are utilized that have been transfected with expression vectors comprising DNA that encodes a voltage-gated ion channel. Preferably, the cells do not naturally express corresponding voltage-gated ion channels. For example, if the expression vectors direct the expression of a voltage-gated calcium channel, the cells will not naturally express voltage-gated calcium channels. Alternatively, if the cells naturally express corresponding voltage-gated ion channels, those corresponding voltage-gated ion channels can be distinguished from the transfected voltage-gated ion channels in some manner, e.g., by the use of appropriate inhibitors, by manipulation of membrane potential. A preferred cell line for use in the present invention is the HEK293 cell line (ATCC 1573) since this cell line naturally expresses endogenous potassium channels, which may be beneficial for electrical field stimulation experiments with channels that cause membrane potential depolarization (e.g., sodium or calcium channels).

Cells are generally eukaryotic cells, preferably mammalian cells. The cells may be grown to the appropriate number on the substrates or they may be placed on the substrate and used without further growth. The cells may be attached to the substrate or, in those embodiments where the cells are placed or grown in wells, the cells may be suspension cells that are suspended in the fluid in the wells. Primary cells or established cell lines may be used.

Suitable cells for transfection with expression vectors that direct the expression of voltage-gated ion channels include but are not limited to cell lines of human, bovine, porcine, monkey and rodent origin. The cells may be adherent or non-adherent. Cells and cell lines which are suitable and which are widely available, include but are not limited to: L cells L-M(TK⁻) (ATCC CCL 1.3), L cells L-M (ATCC CCL 1.2), BEK293 (ATCC CRL 1573), Raji (ATCC CCL 86), CV-1 (ATCC CCL 70), COS-1 (ATCC CRL 1650), COS-7 (ATCC CRL 1651), CHO-K1 (ATCC CCL 61), 3T3 (ATCC CCL 92), NIH/3T3 (ATCC CRL 1658), HeLa (ATCC CCL 2), C127I (ATCC CRL 1616), BS-C-1 (ATCC CCL 26), MRC-5 (ATCC CCL 171), CPAE (ATCC CCL 209), Saos-2 (ATCC IB-85), ARPE-19 human retinal pigment epithelium (ATCC CRL-2302), GH3 cells, and primary cardiac myocytes.

A variety of voltage-gated ion channels may be used in the present invention. For example, voltage-gated sodium channels, voltage-gated potassium channels, and voltage-gated calcium channels are suitable.

In certain embodiments of the present invention, the cells used do not naturally express the voltage-gated ion channel of interest. Instead, DNA encoding the voltage-gated ion channel is transfected into cells in order to express the voltage- gated ion channel in the plasma membrane of the cells. DNA encoding voltage-gated ion channels can be obtained by methods well known in the art. For example, a cDNA fragment encoding a voltage-gated ion channel can be isolated from a suitable cDNA library by using the polymerase chain reaction (PCR) employing suitable primer pairs. The cDNA fragment encoding the voltage-gated ion channel can then be cloned into a suitable expression vector. Primer pairs can be selected based upon the known DNA sequence of the voltage-gated ion channel it is desired to obtain. Suitable cDNA libraries can be made from cellular or tissue sources known to contain mRNA encoding the voltage-gated ion channel.

One skilled in the art would know that for certain voltage-gated ion channels, it is desirable to transfect, and thereby express, more than one subunit in order to obtain a functional voltage-gated ion channel. For example, N-type calcium channels are composed of a multisubunit complex containing at least an α1B, an α2δ, and a β1 subunit. On the other hand, T-type calcium channels are functional with only a single subunit, e.g., α1G, α1H, or α1I. Common knowledge in the art of the subunit composition of a voltage-gated ion channel of interest will lead the skilled artisan to express the correct subunits in the transfected cells.

One skilled in the art could use published voltage-gated ion channel sequences to design PCR primers and published studies of voltage-gated ion channel expression to select the appropriate sources from which to make cDNA libraries in order to obtain DNA encoding the voltage-gated ion channels. The following publications may be of use in this regard:

U.S. Pat. No. 5,380,836 describes nucleic acid sequences encoding a rat cardiac voltage-gated sodium channel;

U.S. Pat. No. 6,030,810 describes a number of voltage-gated, tetrodotoxin-sensitive sodium channels;

U.S. Pat. No. 6,184,349 B1 discloses a human tetrodotoxin-resistant peripheral nerve voltage-gated sodium channel known as PN3; see also GenBank accession no. AF117907;

Isom et al., 1994, Neuron 12:1183-1194 discloses a rat voltage-gated sodium channel β subunit;

McClatchey et al., 1993, Hum. Molec. Gen. 2:745-749 discloses a human voltage-gated sodium channel β1 subunit (hSCNβ1);

Isom et al., Science, 1992, 256:839-842 discloses a rat brain voltage- gated sodium channel β1 subunit (rSCNβ1);

Misgeld et al., 1995, Prog. Neurobiol. 46:423-462; North, 1989, Br. J. Pharmacol. 98:13-23; Gahwiler et al., 1985, Proc. Natl. Acad. Sci USA 82:1558-1562; and Andrade et al., 1986, Science 234:1261-1265 disclose inwardly rectifying voltage-gated potassium channels that are suitable for use in the methods of the present invention.

U.S. Pat. No. 5,874,236 and U.S. Pat. No. 5,429,921 describe various α1 and β subunits of human voltage-gated calcium channels;

U.S. Pat. No. 5,407,820 and U.S. Pat. No. 5,710,250 describe α2 subunits of human voltage-gated calcium channels;

International Patent Publication WO 98/13490 describes a brain- specific P/Q-type human voltage-gated calcium channel involved in familial hemiplagic migraine;

Table 1 provides a list of ion channel genes that are suitable for use in the present invention. TABLE 1 Some ion channel genes of interest for EFS experiments Cytogenetic MIM PubMed Symbol Full Name Location Number ID SCN1 symbol withdrawn, see SCN1A SCN1A sodium channel, voltage-gated, type I, 2q24 182389 8062593 alpha polypeptide SCN1B sodium channel, voltage-gated, type I, beta 19 600235 8394762 polypeptide SCN2A1 sodium channel, voltage-gated, type II, 2q22-q23 182390 1317301 alpha 1 polypeptide SCN2A2 sodium channel, voltage-gated, type II, 2q23-q24 601219 1317301 alpha 2 polypeptide SCN2A symbol withdrawn, see SCN2A1 — SCN2B sodium channel, voltage-gated, type II, 11q22-qter 601327 10198179 beta polypeptide SCN3A sodium channel, voltage-gated, type III, 2q24 182391 9589372 alpha polypeptide SCN4A sodium channel, voltage-gated, type IV, 17q23-q25.3 603967 1654742 alpha polypeptide SCN4B sodium channel, voltage-gated, type IV, reserved beta polypeptide SCN5A sodium channel, voltage-gated, type V, 3p21 600163 alpha polypeptide (long (electrocardiographic) QT syndrome 3) SCN6A sodium channel, voltage-gated, type VI, 2q21-q23 182392 10198179 alpha polypeptide SCN7A symbol withdrawn, see SCN6A — SCN8A sodium channel, voltage gated, type VIII, 12q13.1 600702 7670495 alpha polypeptide SCN9A sodium channel, voltage-gated, type IX, 2q24 603415 7720699 alpha polypeptide SCN10A sodium channel, voltage-gated, type X, 3p21-p22 604427 9839820 alpha polypeptide SCN11A sodium channel, voltage-gated, type XI, 3p21-p24 604385 10444332 alpha polypeptide SCN12A sodium channel, voltage-gated, type XII, 3p23-p21.3 10623608 alpha polypeptide SCNN1 symbol withdrawn, see SCNN1A — SCNN1A sodium channel, nonvoltage-gated 1 alpha 12p13 600228 7896277 SCNN1B sodium channel, nonvoltage-gated 1, beta 16p12.2-p12.1 600760 (Liddle syndrome) SCNN1D sodium channel, nonvoltage-gated 1, delta 1p36.3-p36.2 601328 8661065 SCNN1G sodium channel, nonvoltage-gated 1, 16p12 600761 7490094 gamma CACNA1A calcium channel, voltage-dependent, P/Q 19p13 601011 8825650 type, alpha 1A subunit CACNA1B calcium channel, voltage-dependent, L 9q34 601012 8825650 type, alpha 1B subunit CACNA1C calcium channel, voltage-dependent, L 12pter-p13.2 114205 1650913 type, alpha 1C subunit CACNA1D calcium channel, voltage-dependent, L 3p14.3 114206 1664412 type, alpha 1D subunit CACNA1E calcium channel, voltage-dependent, alpha 1q25-q31 601013 8388125 1E subunit CACNA1F calcium channel, voltage-dependent, alpha Xp11.23-p11.22 300110 9344658 1F subunit CACNA1G calcium channel, voltage-dependent, alpha 17q22 604065 9495342 1G subunit CACNA1H calcium channel, voltage-dependent, alpha 16p13.3 9670923 1H subunit CACNA1I calcium channel, voltage-dependent, alpha 22q12.3-13.2 10454147 1I subunit CACNA1S calcium channel, voltage-dependent, L 1q31-q32 114208 7916735 type, alpha 1S subunit CACNA2 symbol withdrawn, see CACNA2D1 — CACNA2D1 calcium channel, voltage-dependent, alpha 7q21-q22 114204 8188232 2/delta subunit 1 CACNA2D2 calcium channel, voltage-dependent, alpha reserved 2/delta subunit 2 CACNB1 calcium channel, voltage-dependent, beta 1 17q21-q22 114207 8381767 subunit CACNB2 calcium channel, voltage-dependent, beta 2 10p12 600003 9254841 subunit CACNB3 calcium channel, voltage-dependent, beta 3 12q13 601958 8119293 subunit CACNB4 calcium channel, voltage-dependent, beta 4 2q22-q31 601949 9628818 subunit CACNG1 calcium channel, voltage-dependent, 17q24 114209 8395940 gamma subunit 1 CACNG2 calcium channel, voltage-dependent, reserved 602911 gamma subunit 2 CACNG3 calcium channel, voltage-dependent, reserved gamma subunit 3 CACNG4 calcium channel, voltage-dependent, 17q24 10613843 gamma subunit 4 CACNG5 calcium channel, voltage-dependent, 17q24 10613843 gamma subunit 5 CACNG6 calcium channel, voltage-dependent, 19q13.4 11170751 gamma subunit 6 CACNG7 calcium channel, voltage-dependent, 19q13.4 11170751 gamma subunit 7 CACNG8 calcium channel, voltage-dependent, 19q13.4 11170751 gamma subunit 8 KCNA1 potassium voltage-gated channel, shaker- 12p13 176260 1349297 related subfamily, member 1 (episodic ataxia with myokymia) KCNA1B literature alias, see KCNAB1 — KCNA2 potassium voltage-gated channel, shaker- 12 176262 related subfamily, member 2 KCNA2B literature alias, see KCNAB2 — KCNA3 potassium voltage-gated channel, shaker- 1p13.3 or 13 176263 2251283 related subfamily, member 3 KCNA3B literature alias, see KCNAB3 — KCNA4 potassium voltage-gated channel, shaker- 11p14 176266 2263489 related subfamily, member 4 KCNA4L potassium voltage-gated channel, shaker- 11q14 8449523 related subfamily, member 4-like KCNA5 potassium voltage-gated channel, shaker- 12 176267 related subfamily, member 5 KCNA6 potassium voltage-gated channel, shaker- reserved 176257 related subfamily, member 6 KCNA7 potassium voltage-gated channel, shaker- 19 176268 related subfamily, member 7 KCNA8 literature alias, see KCNQ1 — KCNA9 symbol withdrawn, see KCNQ1 — KCNA10 potassium voltage-gated channel, shaker- reserved 602420 related subfamily, member 10 KCNAB1 potassium voltage-gated channel, shaker- 3q26.1 601141 8838324 related subfamily, beta member 1 KCNAB2 potassium voltage-gated channel, shaker- 1p36.3 601142 8838324 related subfamily, beta member 2 KCNAB3 potassium voltage-gated channel, shaker- 17p13.1 604111 9857044 related subfamily, beta member 3 KCNB1 potassium voltage-gated channel, Shab- 20q13.2 600397 7774931 related subfamily, member 1 KCNB2 potassium voltage-gated channel, Shab-  8 9612272 related subfamily, member 2 KCNC1 potassium voltage-gated channel, Shaw- 11p15 176258 8449507 related subfamily, member 1 KCNC2 potassium voltage-gated channel, Shaw- 12 and 176256 8111118 related subfamily, member 2 19q13.4 KCNC3 potassium voltage-gated channel, Shaw- 19 176264 1740329 related subfamily, member 3 KCNC4 potassium voltage-gated channel, Shaw- 1p21 176265 1920536 related subfamily, member 4 KCND1 potassium voltage-gated channel, Shal- Xp11.23-p11.3 300281 10729221 related subfamily, member 1 KCND2 potassium voltage-gated channel, Shal- 7q31-32 605410 10551270 related subfamily, member 2 KCND3 potassium voltage-gated channel, Shal- 1p13.2 605411 10942109 related subfamily, member 3 KCNE1 potassium voltage-gated channel, Isk- 21q22.1-q22.2 176261 8432548 related family, member 1 KCNE1L potassium voltage-gated channel, Isk- Xq22.3 300328 10493825 related family, member 1-like KCNE2 potassium voltage-gated channel, Isk- 21q22.1 603796 10219239 related family, member 2 KCNE3 potassium voltage-gated channel, Isk- reserved 604433 10219239 related family, member 3 KCNE4 potassium voltage-gated channel, Isk- reserved 10219239 related family, member 4 KCNF1 potassium voltage-gated channel, 2p25 603787 9434767 subfamily F, member 1 KCNF2 literature alias, see KCNG2 — KCNF symbol withdrawn, see KCNF1 — KCNG1 potassium voltage-gated channel, 20q13 603788 9434767 subfamily G, member 1 KCNG2 potassium voltage-gated channel, 18q22-18q23 605696 10551266 subfamily G, member 2 KCNG symbol withdrawn, see KCNG1 — KCNH1 potassium voltage-gated channel, 1q32-41 603305 9738473 subfamily H (eag-related), member 1 KCNH2 potassium voltage-gated channel, 7q35-q36 152427 7842012 subfamily H (eag-related), member 2 KCNH3 potassium voltage-gated channel, 12q13 604527 10455180 subfamily H (eag-related), member 3 KCNH4 potassium voltage-gated channel, reserved 604528 10455180 subfamily H (eag-related), member 4 KCNH5 potassium voltage-gated channel, 14 605716 9738473 subfamily H (eag-related), member 5 KCNIP1 Kv channel interacting protein 1 reserved 10676964 KCNIP2 Kv channel-interacting protein 2 10 10676964 KCNIP3 literature alias, see CSEN — KCNJ1 potassium inwardly-rectifying channel, 11q24 600359 7680431 subfamily J, member 1 KCNJ2 potassium inwardly-rectifying channel, 17q23.1-q24.2 600681 7696590 subfamily J, member 2 KCNJ3 potassium inwardly-rectifying channel, 2q24.1 601534 8088798 subfamily J, member 3 KCNJ4 potassium inwardly-rectifying channel, 22q13.1 600504 8016146 subfamily J, member 4 KCNJ5 potassium inwardly-rectifying channel, 11q24 600734 subfamily J, member 5 KCNJ6 potassium inwardly-rectifying channel, 21q22.1 600877 7796919 subfamily J, member 6 KCNJ7 symbol withdrawn, see KCNJ6 — KCNJ8 potassium inwardly-rectifying channel, 12p11.23 600935 8595887 subfamily J, member 8 KCNJ9 potassium inwardly-rectifying channel, 1q21-1q23 600932 8575783 subfamily J, member 9 KCNJ10 potassium inwardly-rectifying channel, 1q 602208 9367690 subfamily J, member 10 KCNJ11 potassium inwardly-rectifying channel, 11p15.1 600937 7502040 subfamily J, member 11 KCNJ12 potassium inwardly-rectifying channel, 17p11.1 602323 7859381 subfamily J, member 12 KCNJ13 potassium inwardly-rectifying channel, 2q37 603208 9878260 subfamily J, member 13 KCNJ14 potassium inwardly-rectifying channel, 19q13 603953 9592090 subfamily J, member 14 KCNJ15 potassium inwardly-rectifying channel, 21q22.2 602106 9299242 subfamily J, member 15 KCNJ16 potassium inwardly-rectifying channel, 17q23.1-q24.2 605722 11240146 subfamily J, member 16 KCNJN1 channel, subfamily J, inhibitor 1 17p11.2-p11.1 602604 8647284 KCNK1 potassium channel, subfamily K, member 1q42-q43 601745 8661042 1 (TWIK-1) KCNK2 potassium channel, subfamily K, member 1q41 603219 9721223 2 (TREK-1) KCNK3 potassium channel, subfamily K, member 2p23 603220 9312005 3 (TASK-1) KCNK4 potassium inwardly-rectifying channel, 11q13 605720 10767409 subfamily K, member 4 KCNK5 potassium channel, subfamily K, member 6p21 603493 9812978 5 (TASK-2) KCNK6 potassium channel, subfamily K, member 19q13.1 603939 10075682 6 (TWIK-2) KCNK7 potassium channel, subfamily K, member 7 11q13 603940 10206991 KCNK9 potassium channel, subfamily K, member  8 605874 10734076 9 (TASK-3) KCNK10 potassium channel, subfamily K, member reserved 605873 10 KCNK12 potassium channel, subfamily K, member 2p22-2p21 12 KCNK13 potassium channel, subfamily K, member 14q24.1-14q24.3 11060316 13 KCNK14 potassium channel, subfamily K, member 2p22-2p21 11060316 14 KCNK15 potassium channel, subfamily K, member reserved 15 KCNMA1 potassium large conductance calcium- 10 600150 7987297 activated channel, subfamily M, alpha member 1 KCNMB1 potassium large conductance calcium- 5q34 603951 8799178 activated channel, subfamily M, beta member 1 KCNMB2 symbol withdrawn, see KCNMB3 — KCNMB2 potassium large conductance calcium- reserved 605214 10097176 activated channel, subfamily M, beta member 2 KCNMB2L symbol withdrawn, see KCNMB3L — KCNMB3 potassium large conductance calcium- 3q26.3-q27 605222 10585773 activated channel, subfamily M beta member 3 KCNMB3L potassium large conductance calcium- 22q11 10585773 activated channel, subfamily M, beta member 3-like KCNMB4 potassium large conductance calcium- reserved 605223 activated channel, subfamily M, beta member 4 KCNMBL symbol withdrawn, see KCNMB3 — KCNMBLP symbol withdrawn, see KCNMB3L — KCNN1 potassium intermediate/small conductance 19p13.1 602982 8781233 calcium-activated channel, subfamily N, member 1 KCNN2 potassium intermediate/small conductance reserved 605879 calcium-activated channel, subfamily N, member 2 KCNN3 potassium intermediate/small conductance 22q11-q13.1 602983 9491810 calcium-activated channel, subfamily N, member 3 KCNN4 potassium intermediate/small conductance 19q13.2 602754 9380751 calcium-activated channel, subfamily N, member 4 KCNQ1 potassium voltage-gated channel, KQT- 11p15.5 192500 8528244 like subfamily, member 1 KCNQ1OT1 KCNQ1 overlapping transcript 1 11p15.5 604115 10220444 KCNQ2 potassium voltage-gated channel, KQT- 20q13.3-2 121200 9425895 like subfamily, member 2 20q13.3 KCNQ3 potassium voltage-gated channel, KQT- 8q24 121201 9425900 like subfamily, member 3 KCNQ4 potassium voltage-gated channel, KQT- 1p34 603537 10025409 like subfamily, member 4 KCNQ5 potassium voltage-gated channel, KQT- 6q14 10787416 like subfamily, member 5 KCNS1 potassium voltage-gated channel, delayed- reserved 602905 9305895 rectifier, subfamily S, member 1 KCNS2 potassium voltage-gated channel, delayed- 8q22 602906 9305895 rectifier, subfamily S, member 2 KCNS3 potassium voltage-gated channel, delayed- reserved 603888 10484328 rectifier, subfamily S, member 3

PCR reactions can be carried out with a variety of thermostable enzymes including but not limited to AmpliTaq, AmpliTaq Gold, or Vent polymerase. For AmpliTaq, reactions can be carried out in 10 mM Tris-Cl, pH 8.3, 2.0 mM MgCl₂, 200 μM of each dNTP, 50 mM KCl, 0.2 μM of each primer, 10 ng of DNA template, 0.05 units/μl of AmpliTaq. The reactions are heated at 95° C. for 3 minutes and then cycled 35 times using suitable cycling parameters, including, but not limited to, 95° C., 20 seconds, 62° C., 20 seconds, 72° C., 3 minutes. In addition to these conditions, a variety of suitable PCR protocols can be found in PCR Primer, A Laboratory Manual, edited by C. W. Dieffenbach and G. S. Dveksler, 1995, Cold Spring Harbor Laboratory Press; or PCR Protocols: A Guide to Methods and Applications, Michael et al., eds., 1990, Academic Press.

It is desirable to sequence the DNA encoding voltage-gated ion channels obtained by the herein-described methods, in order to verify that the desired voltage-gated ion channel has in fact been obtained and that no unexpected changes have been introduced into its sequence by the PCR reactions. The DNA can be cloned into suitable cloning vectors or expression vectors, e.g., the mammalian expression vector pcDNA3.1 (Invitrogen, San Diego, Calif.) or other expression vectors known in the art or described herein.

A variety of expression vectors can be used to recombinantly express DNA encoding voltage-gated ion channels for use in the present invention. Commercially available expression vectors which are suitable include, but are not limited to, pMC1neo (Stratagene), pSG5 (Stratagene), pcDNAI and pcDNAIamp, pcDNA3, pcDNA3.1, pCR3.1 (Invitrogen, San Diego, Calif.), EBO-pSV2-neo (ATCC 37593), pBPV-1(8-2) (ATCC 37110), pdBPV-MMTneo(342-12) (ATCC 37224), pRSVgpt (ATCC 37199), pRSVneo (ATCC 37198), pCI.neo (Promega), pTRE (Clontech, Palo Alto, Calif.), pV1Jneo, pIRESneo (Clontech, Palo Alto, Calif.), pCEP4 (Invitrogen, San Diego, Calif.), pSC11, and pSV2-dhfr (ATCC 37146). The choice of vector will depend upon cell type in which it is desired to express the voltage-gated ion channels, as well as on the level of expression desired, and the like.

The expression vectors can be used to transiently express or stably express the voltage-gated ion channels. The transient expression or stable expression of transfected DNA is well known in the art. See, e.g., Ausubel et al., 1995, “Introduction of DNA into mammalian cells,” in Current Protocols in Molecular Biology, sections 9.5.1-9.5.6 (John Wiley & Sons, Inc.).

As an alternative to the above-described PCR methods, cDNA clones encoding ion channels can be isolated from cDNA libraries using as a probe oligonucleotides specific for the desired voltage-gated ion channels and methods well known in the art for screening cDNA libraries with oligonucleotide probes. Such methods are described in, e.g., Sambrook et al., 1989, Molecular Cloning: A Laboratory Manual; Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.; Glover, D. M. (ed.), 1985, DNA Cloning: A Practical Approach, MRL Press, Ltd., Oxford, U.K., Vol. I, II. Oligonucleotides that are specific for particular voltage- gated ion channels and that can be used to screen cDNA libraries can be readily designed based upon the known DNA sequences of the voltage-gated ion channels and can be synthesized by methods well-known in the art.

The present invention also provides apparatuses for use with the methods disclosed herein. For example, the present invention provides a multiwell tissue culture plate where a plurality of the wells of the plate contain a pair of electrodes disposed such that when a preselected voltage is applied across the electrodes the transmembrane potential of cells within the wells is altered.

In certain embodiments, the multiwell tissue culture plate contains one of the pair of electrodes on the bottom of the wells and the other of the pair of electrodes on the side of the wells. This embodiment is depicted in FIG. 2B.

In other embodiments, the multiwell tissue culture plate contains both of the pair of electrodes on the bottom of the wells. This embodiment is depicted in FIG. 2C.

In other embodiments of the multiwell tissue culture plate, one of the pair of electrodes is a layer of conductive material that forms the bottom of the wells and the other of the pair of electrodes enters the wells from above. This embodiment is depicted in FIGS. 7, 12, and 16.

In other embodiments of the multiwell tissue culture plate, both of the pair of electrodes are embedded in an insulator and enter the wells from above. This embodiment is depicted in FIGS. 9 and 10.

In other embodiments of the multiwell tissue culture plate, the electrode that enters the wells from above has a central conductive material portion that is surrounded by an insulator. This embodiment is depicted in FIG. 8.

In other embodiments of the multiwell tissue culture plate, one of the pair of electrodes forms the bottom of the wells and the other of the pair of electrodes enters the wells from above. This embodiment is depicted in FIGS. 7 and 10.

In other embodiments of the multiwell tissue culture plate, the pairs of electrodes form an alternating pattern of positive and negative electrodes in the wells. This embodiment is depicted in FIG. 16.

In other embodiments of the multiwell tissue culture plate, the layer of conductive material that forms the bottom of the wells is a layer of indium tin oxide that overlays a glass substrate. Preferably, the layer of conductive material and the glass substrate are transparent.

In other embodiments of the multiwell tissue culture plate, a plurality of the wells of the plate contain interdigitating electrodes. This embodiment is depicted in FIGS. 3 and 5.

The present invention provides a multiwell tissue culture plate where:

-   -   the bottom of the wells is a filter membrane upon which cells         can be grown;     -   the wells are located in a trough that can contain fluid;     -   the trough contains a first electrode;     -   a second electrode enters the wells from above;         where the first and second electrodes are so disposed that when         a preselected voltage is applied across the electrodes the         transmembrane potential of cells within the wells is altered.         This embodiment is depicted in FIG. 8.

The present invention also provides a combination of the multiwell tissue culture plates disclosed herein and a fluorescent imager where the multiwell tissue culture plate and the fluorescent imager are positioned relative to one another such that the fluorescent imager can obtain fluorescent readings from the wells of the multiwell tissue culture plate.

The present invention also provides a combination of a top substrate and a bottom substrate where the top and bottom substrates each contain:

-   -   a plurality of virtual wells; and     -   a layer of conductive material that forms the bottoms of the         virtual wells; where the layers of conductive material in the         top and bottom substrates are connected to a pulse generator         such that the layers of conductive material function as         electrodes such that when a preselected voltage is applied         across the electrodes the transmembrane potential of cells         within the virtual wells is altered. Such a combination is         depicted in FIGS. 6 and 13.

The present invention also provides a substrate having square or rectangular wells formed by a plurality of generally parallel positive and negative electrodes and a plurality of spacers arranged generally at right angles to the electrodes, where:

-   one wall of the wells is formed by a positive electrode and the     opposite wall of the well is formed by a negative electrode; -   the spacers form the walls of the wells that are at right angles to     the walls formed by the electrodes; -   where the electrodes are so disposed that when a preselected voltage     is applied across the electrodes the transmembrane potential of     cells within the wells is altered. Such a substrate is depicted in     FIG. 1.

An example of another embodiment of the present invention comprises:

-   -   a substrate having an upper surface upon which are present at         least 10³ living eukaryotic cells which have a voltage-gated ion         channel of interest in their plasma membranes;     -   a plurality of positive electrodes and a plurality of negative         electrodes positioned either on or near the substrate such that         when a voltage is applied through the positive and negative         electrodes the transmembrane potential of the cells is altered;     -   at least one substance that is suspected of being an activator         or an inhibitor of the voltage-gated ion channel;     -   where the cells contain a fluorescent indicator compound.

An example of another embodiment of the present invention comprises:

-   -   a multiwell tissue culture plate having a plurality of wells in         which are present at least 10³ living eukaryotic cells per well         of the plurality which cells have a voltage-gated ion channel of         interest in their plasma membranes;     -   a plurality of positive electrodes and a plurality of negative         electrodes positioned such that when a preselected voltage is         applied through the positive and negative electrodes, the         transmembrane potential of the cells is altered;     -   at least one substance that is suspected of being an activator         or an inhibitor of the voltage-gated ion channel in at least one         of the plurality of the wells;     -   where the cells contain a fluorescent indicator compound or a         voltage sensitive membrane dye.

The following non-limiting examples are presented to better illustrate the invention.

EXAMPLE 1

In FIG. 24, a preferred system for conducting high throughput screening using EFS stimulation is shown. The system consist of a computer 2402 that comprises an arbitrary waveform generator card 2404 electronically associated with the computer 2402. Custom software was written on the computer 2402 which causes the arbitrary generator card 2404 to generate a pulse voltage waveform (2406) of the appropriate electrical stimulus. The voltage waveform (2406) is applied to the input of eight constant current amplifiers 2408. Each constant current amplifier 2408 services a row on the 96-well sample filter plate 2410. The outputs from the amplifiers 2412 pass through the contacts of electrical relays 2414 allowing the current pulse to be applied to the electrodes 2416.

The waveform generator card 2404 also generates a 7-bit binary transistor-transistor logic TM value (2418) that represents the address of the well to be excited by the stimulus. In addition, a trigger pulse 2420 is generated. Microprocessor controller 2422, waits for the trigger pulse 2420, interprets the binary value (2418) and then switches on the appropriate relay 2414 which then directs the constant current pulse (2424) to the particular electrode 2416 or electrodes, via electrode connecting wire(s) 2417 in the sample well 2426. Current flows from the amplifier's output (2424), through the relay contact 2414 through the electrode 2416 the liquid in the well 2428, through the well's membrane 2430 and returns via fluid 2432 beneath the membrane 2430 and a return wire 2434. One large common current return trough 2436 services all 96-electrodes. Other arrangements are possible where each sample well has its own isolated current return trough and return wire. See Example 2 below.

The current return trough 2436 beneath the membranes 2430 has a clear glass bottom 2438 that permits excitation light (2440) from a light source 2442 to pass through the glass bottom 2438, through the transparent membrane 2430 and illuminate cells 2444 adhered to the membrane 2430. Fluorescent light (2446) from the cells 2444 returns back through the membrane 2430 and the glass bottom 2438 entering into the detector 2448. Suitable detectors include those described supra. The preferred detector is the FLIPR (Molecular Devices) fluorescence imager.

When the pulse sequence is completed, the microprocessor controller 2422 switches off the relays 2414 isolating the constant current amplifiers' pulses (2424) from the electrodes 2416.

Turning to FIGS. 25 and 26, FIG. 25 represents a photograph of an electrode head 2500 embodiment comprising top electrodes 2516 and first electrode connecting wires 2517. The electrode head comprises a ground contact rod 2510. FIG. 26 represents a photograph of a trough embodiment 2600 for use in conjunction with the electrode head 2500 embodiment shown in FIG. 25. The trough 2600 comprises bracing posts 2610 to assist in aligning and attachment of the electrode head through apertures 2520 in the electrode head 2500 (see FIG. 28). A bottom electrode wire (hidden) is positioned in the trough which when submerged in the salt/buffer solution, upon assembly of the EFS system (see FIG. 28) acts as bottom electrode for each of the wells. The bottom electrode wire is in electrical communication with a return connection wire 2620 at position 2630. The return connection wire is secured to the ground contact rod 2510 upon assembly of the EFS system. The trough 2600 also comprises a transparent bottom portion 2640 preferably made of glass.

FIG. 27 represents a photograph of the trough embodiment 2600 wherein a Multiscreen™-Black CM 96 wellplate 2700, with 96 wells 2710, is positioned in the trough 2600. Information concerning Millipore's multiscreen plates and biopore membranes is found, e.g., at http://www.millipore.com/catalogue.nsf/docs/C7781 and http://www.millipore.com/publications.nsf/docs/tn062.

FIG. 28 is a photograph of the assembled EFS system 2800 comprising the trough 2600 with well plate 2700 in place. The electrode head 2500 is secured to the top of the trough 2600 such that the electrodes 2416 are inserted into the wells 2710, one electrode per well. The electrode head 2500 is secured down onto bracing posts 2610 (hidden) by fasteners 2810. The fasteners are preferable threaded nuts. Preferably, prior to assembly, each well 2710 (hidden) has been loaded with cells which have been cultured to canvas the bottom of the wells 2710 (hidden). After cells have been cultured under standard and known conditions, and before assembly of the EFS unit 2800, each well is preferably washed to remove cell media and then loaded with the predetermined buffer solution as discussed above.

FIG. 29 shows a graphical representation of data obtained from an embodiment of the invention similar to that depicted in FIG. 28. The data represent a membrane potential change in HEK293 cells that have been transfected to express human PN1 voltage-gated sodium channel. Each plot represents a row (12wells) A-H of a 96-well plate. Each column of the 96-well plate data was acquired for 15 seconds on a VIPR™. Stimulation pulse protocol was applied during the data acquisition as follows; 2 s baseline was followed with a 2 ms square pulse, Amplitude=20 mA, Frequency=10 Hz, Duration=5 s. Those skilled in the art will readily appreciate, in view of the teachings herein, that the subject system may generate a pulse between 1 μs to is. Preferably, the pulse generated is between about 0.1 ms and about 100 ms.

FIG. 30 is a bar graph representation of the peak ratio change of data depicted in FIG. 29. 1 μM TTX a specific and potent blocker of tetrodotoxin (TTX)- sensitive voltage-gated sodium channels is present in wells E1, F1, G1, H1, A12, B12, C12 and D12. In addition well A11 contains an internal standard for blocking TTX- sensitive voltage-gated sodium channels. Z-score is a measure of the difference in the uninhibited and inhibited signal divided by the sum of the standard deviations.

FIG. 31 shows the effects of increasing concentrations of TTX (upper panel) and of Compound A (lower panel) on the EFS-stimulated depolarization signal in HEK293/PN1 cells. The IC₅₀s obtained in these experiments are comparable to those obtained through other techniques. The high Hill coefficients, nH, result from the threshold nature of the stimulation protocol.

EXAMPLE 2

FIG. 32 represents a photograph of an EFS embodiment 3200 pertaining to an alternative EFS system configuration. The electrode head 2500 is similar to that described above in FIG. 25. However, the configurations of the electrodes, wells and trough are configured differently to further isolate the electrical fields. This reduces cross-talk and interference between wells. For this embodiment, the inventors have adapted Millipore's Multiscreen™ Caco-2 Assay System for use as a EFS system. Information concerning the Multiscreen™ Caco-2 Assay System can be found at http://www.millipore.com/publications.nsf/docs/PF1780EN00. The standard commercially available Caco-2 plate system comprises two plates: a membrane- bottom cell growth plate and a 96-well receiver tray. One of the unique characteristics of the Caco-2 system is that it each well has an individual corresponding trough that is accessed basolaterally to each well. Therefore, it supplants the need for a common trough into which all of the wells sit. According to this embodiment, the top electrodes 2516 are disposed into each of the wells in the membrane-bottom cell growth plate (hidden). To establish the bottom electrode for each well, a conductive electrode plate 3220 is provided. The conductive electrode plate 3220 comprise a series of well apertures 3230, providing access of the top electrodes 2410 into the individual wells during assembly. The conductive electrode plate 3220 also comprises a series of conductive pins (hidden) secured thereto and extending downward at positions 3240. These conductive pins are inserted through the basolateral access port of the membrane-bottom cell growth plate (not shown).

FIG. 33 is a depiction of the bottom of the conductive electrode plate 3220 and shows the conductive pins 3310, which are extending out of the page toward the reader. FIG. 34 shows a side-view of the conductive electrode plate 3220 properly positioned atop of the membrane-bottom cell growth plate 3410 and 96 well receiver tray 3420. When the electrode conductive plate 3220 is properly positioned on top of the membrane-bottom cell growth plate 3410, the conductive pins 3310 are inserted through the basolateral access port (not shown) into the individual trough area (not shown) of the 96 well receiver tray. When the individual trough area is filled with the appropriate solution it contacts the bottom of each well and individual pin. Therefore, when the well and trough area are filled with solution, current may flow from the top electrode to the bottom electrode during operation. FIG. 35 is a side-view of the assembled EFS system. The assembled system comprises the membrane-bottom cell growth plate 3410 positioned in the 96 well tray 3420. The electrode plate 3220 is mounted on top of the membrane bottom well plate 3410. The electrode head 2500 is shown mounted on top of the electrode plate 3220.

One clear advantage to the EFS systems described in Examples 1 and 2 above, and elsewhere in the present application, is the ability to generate a uniform field across the cells, as opposed to tangential to the cells. Generating an electrical field across the cells is made possible by the novel “top to bottom” placement of the electrodes in a multiwell format.

EXAMPLE 3

FIG. 36 shows a novel electrode embodiment 3600. FIG. 36A depicts an expanded view of the electrode 3600. The electrode 3600 comprises two parallel plates 3610 and 3630 with a low dielectric plate or disc 3620 between them. Optionally, the electrode may be coated with an insulating material. Potential advantages of this design are that special multiwell plates are not required, i.e., any plate that the cells will stick to and that the stimulation and emission light will pass through may be used. There is no filter in the well that may absorb compound or pass compound during long incubations. In the case of the coated electrode, very little current is used and ohmic heating is diminished, even for dc current and even for extended periods of stimulation. The capacitance current is low enough that this advantage applies to ac current as well. The sealed electrode permits placement very close to the cell layer for more uniform stimulation.

Not to be bound by any theory, it is believed that the more uniform the electrical field presented to the cells is, a more accurate indication of potential modulation to the cells will be achieved. In other words, the more uniform the electrical field is, the potential modulation as observed by any of the methods presented herein, e.g., fluorescence, will more directly correlate to actual modulation of ion channels in the cell membrane, and less correlate with background noise in the system caused by cross-interference, cross-illumination, dye effects, dye leaching or any other interference in the system. One way to increase the uniformity of the electrical field applied to the cells is to present one or more of the electrodes in close proximity to, or in contact with, the cells. However, this can affect the cells in deleterious ways leading to failure in the system. Some of the problems associated with close proximity or contact of the electrode(s) to the cells are caused by, for example, ohmic heating, oxidation and formation of bubbles on the electrode. The embodiments of the present invention as taught in FIGS. 8, 11, 24-28 and 32-35 are particularly preferred because they achieve a uniform electrical field across the cells without putting the electrodes in contact with or close proximity to the cells. Furthermore, the novel electrode design shown in FIG. 36 achieves a uniform electrical field, by allowing close proximity of the electrode to the cells, without creating the problems of ohmic heating, oxidation, or bubbling of the cells.

It is believed that the subject EFS system embodiments produce substantially uniform fields, where the one or more electrical fields vary over an area of observation by no more than about 30% from the mean electrical field at any one time. Percentages are determined by measurements in two dimensions; or preferably, variation is calculated in three dimensions. In a more preferred embodiment, the one or more electrical fields vary over an area of observation by no more than about 15% from the mean electrical field at any one time. In an even more preferred embodiment, the one or more electrical fields vary over an area of observation by no more than 10% from the mean electrical field at any one time. In an optimal embodiment, the variation is no more than 5% from the mean.

The similarity to a capacitor is obvious, but the low dielectric 3620 between the plates 3610 and 3630 reduces the amount of current required to initially charge the plates with only a miniscule current required to maintain the charge between the plates. An external electric field is generated that can be used to depolarize the cells. The external electric field density is reduced by a high dielectric between the plates as is used with an authentic capacitor and is maximal with a low dielectric such as teflon or mylar or no dielectric. The external field density is further enhanced by placing the plates very close together, but the optimal separation may be determined empirically.

FIG. 36B shows an embodiment comprising a concurrent lead design. The concurrent lead comprises an internal wire 3655 and an external wire 3650. The internal wire passes through the top plate 3610 and dielectric plate 3620 and is attached or integral to the bottom plate 3630. The external wire is attached or integral to the top plate 3610. Those skilled in the art will recognize that the foregoing arrangement of the leads may be reversed. FIG. 36C shows an embodiment comprising edge leads 3660 and 3665. Edge lead 3660 is attached or integral to top plate 3610 and edge lead 3665 is attached or integral to bottom plate 3630.

Some of the embodiments of the subject invention include the following:

A method of characterizing the biological activity of a candidate compound comprising.

-   -   exposing one or more cells to said compound; repetitively         exposing said one or more cells to one or more electric fields         so as to effect a controlled change in transmembrane potential         of said one or more cells; and monitoring, without using a patch         clamp, changes in the transmembrane potential of said one or         more cells.

The above method, where the monitoring comprises detecting fluorescence emission from an area of observation containing said one or more cells.

The above method, where the electric fields are biphasic.

The above method, additionally comprising limiting spatial variation in electric field intensity so as to minimize irreversible cell electroporation.

The above method, where one or more electrical fields may cause an ion channel of interest to cycle between different voltage dependent states.

The above method, where the one or more electrical fields cause an ion channel of interest to open.

The above method, where the one or more electrical fields cause an ion channel of interest to be released from inactivation.

The above method, where the one or more cells comprise a voltage sensor selected from the group consisting of a FRET based voltage sensor, an electrochromic transmembrane potential dye, a transmembrane potential redistribution dye, an ion sensitive fluorescent or luminescent molecule and a radioactive ion.

The above method, where the one or more cells comprise a voltage regulated ion channel.

The above method, where the voltage regulated ion channel is selected from the group consisting of a potassium channel, a calcium channel, a chloride channel and a sodium channel.

The above method, where the electric field exhibits limited spatial variation in intensity in the area of observation of less than about 25% from. a mean intensity in that area.

The above method, where the one or more electrical fields varies over an area of observation by no more than about 15% from the mean electrical field at any one time.

The above method, where the one or more electrical fields varies over an area of observation by no more than about 5% from the mean electrical field at any one time.

The above method, where the one or more electrical fields comprises stimulation with either a square wave-form, a sinusoidal wave-form or a saw tooth wave-form.

The above method, where the one or more electrical fields have an amplitude within the range of about 10 V/cm to about 100 V/cm.

The above method, where the one or more electrical fields have an amplitude within the range of about 20 V/cm to about 80 V/cm.

The above method, where the one or more electrical fields are repeated at a frequency of stimulation that is greater than or equal to the reciprocal of the transmembrane time constant of said one or more cells.

The above method, where the one or more electrical fields are repeated at a frequency of stimulation within the range of zero to 1 kHz.

The above method, where the one or more electrical fields have a pulse duration within the range of about 100 microseconds to about 20 milliseconds.

The above method, where the transmembrane potential is developed across the plasma membrane of said one or more cells.

A method of assaying the biochemical activity of a compound against a target ion channel comprising.

-   -   selecting a cell line having a normal resting transmembrane         potential corresponding to a selected voltage dependent state of         said target ion channel; expressing said target ion channel in a         population of cells of said selected cell line; exposing said         population of cells to said compound; repetitively exposing said         population of cells to one or more electric fields so as to         effect a controlled change in transmembrane potential of said         one or more cells; and monitoring changes in the transmembrane         potential of said one or more cells.

The above method, where the target ion channel is exogenously expressed in said cell line.

The above method, where the cell line is transfected with nucleic acid encoding said target ion channel.

The above method, where the cell line expresses insignificant levels of other ion channels.

The above method, where the cell line is selected from the group consisting of CUL,LTK(−), and CHO-M.

The above method, where the target ion channel is a sodium channel, and wherein said population of cells is selected from the group consisting of CHL cells, LTK(−) cells, and CHO-K1 cells.

The above method, where the target ion channel is a sodium channel, and wherein said population of cells is selected from the group consisting of HEK-293 cells, RBL cells, F11 cells, and HL5 cells.

The above method, where the target ion channel is a potassium channel, and wherein said population of cells is selected from the group consisting of CHL cells, LTK(−) cells, and CHO-K1 cells.

The above method, where the target ion channel is a calcium channel, and wherein said population of cells is selected from the group consisting of CHL cells, LTK(−) cells, and CHO-K1 cells.

A method of assaying ion channel activity comprising.

-   -   exposing at least one cell to a plurality of electric field         pulses so as to create a controlled change in transmembrane         potential and so as to activate an ion channel of interest; and         detecting ion channel activity by detecting one or more changes         in transmembrane potential without using a patch clamp.

The above method, where the at least one cell comprises a voltage sensor selected from the group consisting of a FRET based voltage sensor, an electrochromic transmembrane potential dye, a transmembrane potential redistribution dye, an ion sensitive fluorescent or luminescent molecule and a radioactive ion.

The above method, where the voltage sensor comprises a FRET based voltage sensor.

The above method, where the ion channel of interest is a voltage regulated ion channel.

The above method, where the plurality of electric field pulses cause said ion channel of interest to cycle between different voltage dependent states.

The above method, where the at least one cell is an eukaryotic cell.

The above method, where the at least one cell is a non-excitable cell.

The above method, where the at least one cell is a prokaryotic cell.

The above method, where the at least one cell is a tissue culture cell.

The above method, where the at least one cell is a primary cell line.

The above method, where the at least one cell is part of an intact living organism.

A method of assaying ion channel activity comprising.

-   -   expressing a selected target ion channel in at least one cell;         expressing a selected counter ion channel in said at least one         cell; exposing said at least one cell to a plurality of electric         field pulses so as to create a controlled change in         transmembrane potential and so as to activate said counter ion         channel; and monitoring the transmembrane potential of said at         least one cell.

The above method, where a transmembrane potential change is detected when said ion channel of interest is blocked.

The above method, where the ion channel of interest comprises a ligand gated ion channel.

The above method, where the counter channel comprises a sodium channel.

A method of modifying the transmembrane potential of a cell comprising repetitively applying biphasic electric field pulses to said cell, wherein said pulses have a maximum amplitude of less than approximately 90 V/cm, wherein said pulses are applied at a rate of at least about 1 per second, and wherein the total duration of each pulse is at least about 1 millisecond.

The above method, where the maximum amplitude is approximately 20 to 40 V/cm.

The above method, where the pulse duration is approximately 2 to 10 milliseconds per phase.

The above method, where the pulses are applied at a rate of approximately 20 to 100 pulses per second.

A method of characterizing the biological activity of a candidate compound comprising.

-   -   placing one or more cells into an area of observation in a         sample well; exposing said one or more cells to said compound;         repetitively exposing said one or more cells to a series of         biphasic electric fields at a rate of approximately 20 to 100         pulses per second, wherein said electric fields exhibit limited         spatial variation in intensity in the area of observation of         less than about 25% from a mean intensity in that area, and         wherein said electric fields produce a controlled change in         transmembrane potential of said one or more cells; and         monitoring changes in the transmembrane potential of said one or         more cells by detecting fluorescence emission of a FRET based         voltage sensor from, an area of observation containing said one         or more cells.

The above method, where the one or more electrical fields cause an ion channel of interest to open.

The above method, where the one or more electrical fields cause an ion channel of interest to be released from inactivation.

The above method, where the one or more cells comprise a voltage regulated ion channel.

The above method, where the voltage regulated ion channel is selected from the group consisting of a potassium channel, a calcium channel, a chloride channel and a sodium channel.

The above method, where the one or more electrical fields likely vary over an area of observation by no more than about 15% from the mean electrical field at any one time.

The above method, where the one or more electrical fields varies over an area of observation by no more than about 5% from the mean electrical field at any one time.

The above method, where the one or more electrical fields are selected from a square wave-form, a sinusoidal wave-form or a saw tooth wave-form.

A high throughput screening system comprising.

-   -   a plurality of wells having a high transmittance portion through         which cells present in said wells are optically observable in an         area of observation; two electrodes in each of said plurality of         wells; an optical detector configured to detect light emanating         from said wells through said high transmittance portion; a power         supply connected to said electrodes; wherein said power supply         and said electrodes are configured to apply a series of electric         fields to cells within said area of observation, said electric         fields having a spatial variation of less than about 25% of a         mean field intensity within said area of observation, said         electric fields being effective to controllably alter the         transmembrane potential of a portion of said cells; a data         processing unit configured to interpret said light emanating         from said wells, through said high transmittance portion as ion         channel activity resulting from said transmembrane potential         alterations.

The above high throughput screening system, where the pluarality of wells are located in a multiwell plate.

The above high throughput screening system, where the high transmittance portion is made from a material selected from the group consisting of glass, quartz, cycloolefin, Aclar, polypropylene, polyethylene and polystyrene.

The above high throughput screening system, where the high transmittance portion exhibits less fluorescence when excited with UV light in the range of 250 nm to 400 nm than polystyrene.

The above high throughput screening system, where the electrodes are located in a well of said plurality of wells.

The above high throughput screening system, where the electrodes are located in a bottom layer of said plurality of wells.

The above high throughput screening system, where the multiwell plate comprises up to 96 wells.

The above high throughput screening system, where the multiwell plate comprises greater than 96 wells.

The above high throughput screening system, where the multiwell plate comprises greater than 384 wells.

The above high throughput screening system, where the electrodes are made of a material selected from the group consisting of gold, platinum, palladium, chromium, molybdenum, iridium, tungsten, tantalum and titanium.

The above high throughput screening system, where the multiwell plate comprises optically opaque materials or pigments to reduce the transmission of light.

The above high throughput screening system, where the electrodes are separated by a gap within the range of about 1 to 4 mm.

The above high throughput screening system, where the electrodes are separated by a gap within the range of about 0.1 to 1 mm.

1.0 The above high throughput screening system, where the electrodes are separated by a gap within the range of about 0.01 to 0.1 mm.

The above high throughput screening system, where the electrodes are charged to create an electrical field intensity of between 5 to 100 V/cm across said gap, and wherein the total charge transferred across the surface area of the electrically conductive material, in fluidic connection with the interior of the well is less than or equal to 100 μC/mm2.

The above high throughput screening system, where the plurality of wells further comprise an insulator orientated and configured so as to create an area of observation within said well in which. the electrical field intensity varies by no more than 10% from the mean electrical field intensity when said at least two strips of electrically conductive material are charged to create an electrical field intensity of between 5 to 100 V/cm across said gap, and. wherein the total charge transferred across the surface area of the electrically conductive material, in fluidic connection with the interior of the well is less than or equal to looptC/mm2.

The above high throughput screening system, where the plurality of wells further comprise at least two satellite electrical conductors.

A high throughput screening system comprising.

-   -   sample wells; liquid handling stations for adding reagents         and/or cells to said sample wells; and means for controlling the         transmembrane potential of cells in said sample wells so as to         selectively cause ion channel activity.     -   means for optically monitoring changes in said transmembrane         potential.

The above high throughput screening system, where the means comprises electrodes configured to create an electric field having a spatial variation of less than about 25% of a mean field intensity within an area of observation.

The above high throughput screening system, where the means for controlling the transmembrane potential comprise an electrode array assembly.

The above high throughput screening system, where the electrode assembly array comprises 8 electrode assemblies.

The above high throughput screening system, where the electrode assembly array comprises 96 electrode assemblies.

The above -high throughput screening system, where the electrode assembly array comprises greater than 96 electrode assemblies.

The above high throughput screening system, where the system further comprises means for retractably moving said electrode assembly into and out of the wells of a multiwell plate.

The above high throughput screening system, where the means for controlling the transmembrane potential comprises electrical conductors with two substantially parallel planar surfaces.

The above high throughput screening system, where the electrical conductors are separated by a gap within the range of 1 to 4 mm.

The above high throughput screening system, where the electrical conductors are separated 5 by a gap within the range of 0.1 to 1 mm.

The above high throughput: screening system, where the electrical conductors further comprise a first insulator.

The above high throughput screening system, where the first insulator comprises two planar surfaces orientated perpendicular to said substantially parallel planar surfaces of said electrical conductors and substantially parallel with respect to each other.

The above high throughput: screening system, where the electrical conductors further comprise a second insulator attached to said at least two electrical conductors, wherein said second insulator is interposed in said gap between said at least two electrical conductors to define the depth of said aqueous solution between said at least two electrical conductors.

The above high throughput: screening system, where the first insulator is composed of allow fluorescence material, wherein. said low fluorescence material exhibits less fluorescence when excited with UV light in the range 250 nm to 400 nm than polystyrene of comparable size.

The above high throughput screening system, where the second insulator is composed of a low fluorescence material, wherein said low fluorescence material exhibits less fluorescence when excited with UV light in the range 250 nm to 400 nm than polystyrene of comparable size.

The above high throughput screening system, where the first insulator comprises an insulator selected from the group consisting of plastic, glass and ceramic.

The above high throughput screening system, where the plastic is selected from the group consisting of nylon, polystyrene, Teflon (tetrafluoroethylene), polypropylene, polyethylene,poly-vinyl chloride, and cycloolefin.

The above high throughput screening system, where the electrical conductors comprise a conductor selected from the group consisting of gold, platinum, titanium, tungsten, molybdenum, iridium, vandium, Nb, Ta, stainless steel and graphite.

The above high throughput screening system, where the electrical conductors comprise a surface treatment to reduce electrolysis.

The above high throughput screening system, where the surface treatment to reduce electrolysis comprises platinum black, gold black, iridium/iridium oxide, titanium/titanium nitride or polypyrrole films.

The above high throughput screening system, where the electrical field intensity varies by no more than 10% from the mean electrical field intensity when said at least two electrical conductors are charged to create an electrical field intensity of between 5 to 100 V/cm across said gap, wherein the total charge transferred across the surface area of the electrical conductors in contact with said aqueous solution is less than or equal to 100 μC/mm2.

The above high throughput screening system, where the electrical field intensity varies by no more than 5% from the mean electrical field intensity when said at least two electrical conductors are charged to create an electrical field intensity of between 5 to 100 V/cm across said gap, wherein the total charge transferred across the surface area of the electrical conductors in contact with said aqueous solution is less than or equal to 100 μC/mm2.

A method of screening a plurality of drug candidate compounds against a target ion channel comprising.

-   -   expressing said target ion channel in a population of host         cells; placing a plurality of said host cells into each of a         plurality of sample wells; adding a candidate drug compound to         at least: one of said plurality of sample wells; and modulating         the transmembrane potential of host cells in said plurality of         sample wells with a repetitive application of electric fields so         as to set said transmembrane potential to a level corresponding         to a pre-selected voltage dependent state of said target ion         channel.

The above method, additionally comprising selecting a host: cell line having a normal resting transmembrane potential corresponding to a second pre-selected voltage dependent state of said target ion channel.

The above method, where the electric fields are biphasic.

The above method, where electric fields cause an ion channel of interest to cycle between different voltage dependent states.

The above method, where the electric fields cause an ion channel of interest to open.

The above method, where the electric fields cause an ion channel of interest to be released from inactivation.

The above method, where the one or more cells comprise a voltage sensor selected from the group consisting of a FRET based voltage sensor, an electrochromic transmembrane potential dye, a transmembrane potential redistribution dye, an ion sensitive fluorescent or luminescent molecule and a radioactive ion.

The above method, where the target ion channel is selected from the group consisting of a potassium channel, a calcium channel, a chloride channel and a sodium channel.

The above method, where the one or more electrical fields comprises stimulation with either a square wave-form, a sinusoidal wave-form or a saw tooth wave-form.

The above method, where the one or more electrical fields have an amplitude within the range of about 10 V/cm to about 100 V/cm.

The above method, where the one or more electrical fields have an amplitude within the range of about 20 V/cm to, about 80 V/cm.

An assay plate and electrode assembly comprising at least one sample well having electrodes placed therein, wherein said electrodes are positioned with respect to the bottom surface of the well to provide an electric field adjacent to said bottom surface that varies by less than about 10% from a mean field intensity over at least about 20% of the surface area of said bottom surface.

The above assembly, where the electrodes comprise plate electrodes extending down into said well such that bottom ends of said electrodes are adjacent to but not in contact with said bottom surface.

The above assembly, comprising two electrodes per sample well. The above assembly, comprising more than two electrodes per sample well.

The above assembly, where the electrodes are plated onto said bottom surface of said well. The above assembly, where the bottom surface comprises a high optical transmittance portion.

The above assembly, where the high transmittance portion is made from a material selected from the group consisting of glass, quartz, cycloolefin, Aclar, polypropylene, polyethylene and polystyrene.

The above assembly, where the high transmittance portion exhibits less fluorescence when excited with UV light in the range of 250 nm to 400 nm than polystyrene.

The above assembly, where the electrodes are located in a wall of said plurality of wells.

The above assembly, where the plate comprises up to 96 wells.

The above assembly, where the plate comprises greater than 96 wells.

The above assembly, where the plate comprises greater than 384 wells.

The above assembly, where the electrodes are made of a material selected from the group consisting of gold, platinum, palladium, chromium, molybdenum, iridium, tungsten, tantalum and titanium.

The above assembly, where the electrodes are separated by a gap within the range of about 1 to 4 mm.

The above assembly, where the electrodes are separated by a gap within the range of about 0.1 to 1 mm.

The above assembly, where the electrodes are separated by a gap within the range of about 0.01 to 0.1 mm.

A bottom panel for a multi-well plate comprising.

-   -   at least one row of high transmittance regions with positions         corresponding to well locations; a first: strip of conductive         material extending along said row and overlapping a first         portion of said well locations; and a second strip of conductive         material extending along said row and overlapping a second         portion of said well locations.

The above bottom panel, additionally comprising a first: electrical contact proximate to an end of said first strip and a second electrical contact proximate to an end of said second strip.

An assay apparatus comprising.

-   -   a sample well; a first pair of electrodes positioned within said         sample well; at least one additional satellite electrode         positioned within said sample well.

The above assay apparatus, where the at least one additional satellite electrode comprises second and third pairs of electrodes.

The above assay apparatus, where the satellite electrodes are charged to a potential less than that of the first pair of electrodes.

The above assay apparatus, where the electrodes are positioned with respect to the bottom surface of the well to provide an electric field adjacent to said bottom surface that varies by less than about 10% from a mean field intensity over at least about 20% of the surface area of said bottom surface.

The present invention is not to be limited in scope by the specific embodiments described herein. Indeed, various modifications of the invention in addition to those described herein will become apparent to those skilled in the art from the foregoing description. Such modifications are intended to fall within the scope of the appended claims.

Various publications are cited herein, the disclosures of which are incorporated by reference in their entireties. Furthermore, for general information, PCT Publication No. PCT/US01/21652 is incorporated herein in its entirety to the extent it is accurate and not inconsistent with the teachings herein. All patents, patent applications, publications, texts and references discussed or cited herein are understood to be incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually set forth in its entirety. In addition, all references, patents, applications, and other documents cited in an Invention Disclosure Statement, Examiner's Summary of Cited References, or otherwise entered into the file history of this application are taken to be incorporated by reference into this specification for the benefit of later applications claiming priority to this application. Finally, all terms not specifically defined are first taken to have the meaning given through usage in this disclosure, and if no such meaning is inferable, their normal meaning. 

1-74. (canceled)
 75. A method for identifying modulators of the activity of a voltage-gated ion channel comprising: (a) altering the transmembrane potential of at least a portion of the membrane of a cell expressing the voltage-gated ion channel by applying a voltage to the cells through extracellular electrodes while monitoring ion flow through the voltage-gated ion channel; (b) exposing the cell in step (a) to a substance and monitoring ion flow through the voltage-gated ion channel; (c) comparing the ion flow through the voltage-gated ion channel in step (a) and step (b); where a difference in the ion flow through the voltage-gated ion channel in step (a) and step (b) indicates that the substance is a modulator of the voltage-gated ion channel.
 76. The method of claim 75 wherein a plurality of cells expressing the voltage-gated ion channel in step (a) are divided into a control portion and a test portion and where a difference in the ion flow through the voltage-gated ion channel of the control portion as compared to the test portion indicates that the substance is modulator of the voltage-gated ion channel.
 77. The method of claim 76 wherein a pre-selected voltage is applied through positive and negative electrodes to said plurality of cells in step (a) and where a difference in the ion flow after the pre-selected voltage is applied to the control portion as compared to the test portion indicates that the substance is a modulator of the voltage-gated ion channel.
 78. The method of claim 77 wherein if the value of the control portion is greater than the value of the test portion, then the substance is an inhibitor of the voltage-gated ion channel.
 79. The method of claim 77 wherein if the value of the control portion is less than the value of the test portion, then the substance is an activator of the voltage-gated ion channel.
 80. The method of claim 77 wherein the plurality of cells are on a substrate that is a glass or a multiwell tissue culture plate and is not silicon or a field effect transistor.
 81. The method of claim 80 wherein the multiwell tissue culture plate has a plurality of wells that contain one positive and one negative electrode.
 82. The method of claim 81 wherein the positive or negative electrodes form the bottom of the wells and the other of the positive or negative electrode enters the wells from above.
 83. The method of claim 81 wherein the electrodes are interdigitating.
 84. The method of claim 77 where the cells are selected from the group consisting of: L cells L-M(TK⁻) (ATCC CCL 1.3), L cells L-M (ATCC CCL 1.2), HEK293 (ATCC CRL 1573), Raji (ATCC CCL 86), CV-1 (ATCC CCL 70), COS-1 (ATCC CRL 1650), COS-7 (ATCC CRL 1651), CHO-Ki (ATCC CCL 61), 3T3 (ATCC CCL 92), NIH/3T3 (ATCC CRL 1658), HeLa (ATCC CCL 2), C1271 (ATCC CRL 1616), BS-C-1 (ATCC CCL 26), MRC-5 (ATCC CCL 171), CPAE (ATCC CCL 209), Saos-2 (ATCC HTB-85), ARPE-19 human retinal pigment epithelium (ATCC CRL-2302), GH3 cells, and primary cardiac myocytes.
 85. The method of claim 77 where the cells do not naturally express a voltage-gated ion channel but have been transfected with an expression vector that encodes a voltage-gated ion channel.
 86. The method of claim 77 where the cells contain a fluorescent indicator compound selected from the group consisting of: fluo-3, fura-2, fluo-4, fluo-5, calcium green-1, Oregon green, 488 BAPTA, SNARF-1, and indo-1.
 87. An apparatus for use in identifying activators or inhibitors of voltage-gated ion channels comprising: (a) a substrate having an upper surface upon which are present at least 10³ living eukaryotic cells which have a voltage-gated ion channel of interest in their plasma membranes; (b) a plurality of positive electrodes and a plurality of negative electrodes positioned either on or near the substrate such that when a voltage is applied through the positive and negative electrodes the transmembrane potential of the cells is controlled; (c) at least one substance that is suspected of being an activator or an inhibitor of the voltage-gated ion channel; and (d) where said cells contain a fluorescent indicator compound.
 88. The apparatus of claim 87 wherein: the substrate is a multiwell tissue culture plate having a plurality of wells in which are present at least 10³ living eukaryotic cells per well of the plurality which cells have a voltage-gated ion channel of interest in their plasma membranes; the plurality of positive electrodes and a plurality of negative electrodes are positioned such that when a pre-selected voltage is applied through the positive and negative electrodes, the transmembrane potential of the cells is altered; at least one substance that is suspected of being an activator or an inhibitor of the voltage-gated ion channel is in at least one of the wells; and the cells contain a fluorescent indicator compound or a voltage sensitive membrane dye.
 89. The apparatus of claim 88 wherein a pre-selected voltage applied across the electrodes alters the transmembrane potential of cells within the wells.
 90. The apparatus of claim 88 wherein the multiwell tissue culture plate contains one of the pair of electrodes on the bottom of the wells and the other of the pair of electrodes on the side of the wells.
 91. The apparatus of claim 88 wherein the multiwell tissue culture plate contains both of the pair of electrodes on the bottom of the wells.
 92. The apparatus of claim 88 wherein the wells of the plate contain interdigitating electrodes.
 93. The apparatus of claim 92 wherein the interdigitating electrodes comprise an electrode array that has been chemically etched onto an indium tin oxide coated glass plate.
 94. A system for applying electrical field stimulation to cells, said system comprising: (a) a multiwell tissue culture plate, wherein the bottom of the wells is comprised of an optically transparent filter membrane upon which cells can be grown; (b) a trough suitable for containing fluid and configured such that said multiwell tissue culture plate may sit therein; (c) at least one first electrode disposed in said trough; and (d) an electrode head comprising a plurality of second electrodes in an amount corresponding to the number of wells in said multiwell tissue culture plate, wherein said electrode head and said plurality of electrodes are configured such that said electrodes are disposed in the wells of the multiwell tissue culture plate upon positioning said electrode head onto said multiwell tissue culture plate; and wherein said first electrode and said plurality of second electrodes are so disposed that when a pre-selected voltage is applied across the electrodes the transmembrane potential of cells within the wells is altered.
 95. The system of claim 94 further comprising a waveform generator that is in electrical communication with said first electrode or said plurality of second electrodes, or both, whereby electric pulse signals are generated by said waveform generator.
 96. The system of claim 95 further comprising a computer electrically connected to said waveform generator, said computer comprising software for coordinating said pulse signals produced by said waveform generator.
 97. The system of claim 95 wherein said waveform generator generates a binary value that represents the address of the well to be excited by said pulse signals.
 98. The system of claim 95 further comprising electrical relays upstream of said plurality of second electrodes.
 99. The system of claim 98 further comprising a microcontroller in electrical communication with said waveform generator and said electrical relays so disposed such that upon receiving a trigger pulse and a particular binary value from said waveform generator, said microcontroller switches on the appropriate relay thereby directing a pulse to the particular electrode corresponding to the said particular binary value.
 100. The system of claim 94 wherein: said trough comprises a plurality of individual troughs suitable for containing fluid, where the number of plurality of troughs corresponds to the number of wells in said multiwell tissue culture plate, where the plurality of troughs are so disposed to individually contain each well of said multiwell tissue culture plate; and where said plurality of troughs my be accessed by a port defined in said multiwell tissue culture plate and disposed laterally to each well; and said conductive electrode head comprises a conductive electrode plate configured to be mounted above said multiwell tissue culture plate, wherein said electrode plate comprises a plurality of apertures configured to allow the wells of the multiwell tissue culture plate to pass through said electrode plate, where said electrode plate comprises a plurality of conductive pins integral or attached to said electrode plate, and where the individual pins of said plurality of conductive pins pass through said port to be disposed in individual troughs upon mounting said electrode plate on top of said multiwell tissue culture plate. 